Looks so simple yet my class couldn't figure it out, Reddit r/askmath

Let's figure out the limit of x/(x-1) as x goes to 1. This is a calculus from Reddit r/askmath. See the original post: www.reddit.com...
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#calculus #bprpcalculus #apcalculus #tutorial #math

[@bprpcalculusbasics]

Why doesn’t this limit represent the definition of derivative? 👉kzbin.info/www/bejne/eWWXYaiNiN95fcksi=Zx-wuAEkbfV5_m8a

[@BlacksmithTWD]

This immediately reminded me to when I was thought the limit of 1/x as x goes to 0 to illustrate why one can't divide by 0.

[@winners-r4z]

but if we use L Hospital rule and differentiate the numerator and denominator then we have 1/1-0 which equals 1 so by that method the limit should exist and should be equal to one

[@BlacksmithTWD]

@@winners-r4z Not familiar with the L Hospital rule, So I wonder how you got from x/(x-1) to 1/(1-0) or 1/1-0.

[@mondherbouazizi4433]

​​@@winners-r4z We can only apply the L'Hospital's rule if the direct substitution returns an indeterminate form, that means 0/0 or ±∞/±∞. As bprp said, this is *NOT* an undeterminate form. The limit is clearly ∞, but depending on the direction from which we approach 1, the sign of ∞ changes

[@おす-qz7kp]

Brilliant. I like this kind of videos. Subscribed🎉

[@HalobeatWatcher]

Honestly I think the class didn’t learn limit from right and left or they just forget about it.

[@Lordmewtwo151]

Well, in both cases the function approaches 1/0. However, where x is less than 1 and greater than 0, the function is negative. Likewise, when x is greater than 1 or less than 0 (which is irrelevant to this question), the function is positive.

[@JasperJanssen]

I did learn about that (albeit 25 years ago, oh god, and not with this guy’s notation) and my first reaction was that it doesn’t specify from which side it is in the problem.

[@thetaomegatheta]

@@JasperJanssen Why is that a problem? If the side is not specified, it's obviously the standard, and not a one-sided limit.

[@JasperJanssen]

@@thetaomegatheta … did you watch the video? And no, there is no such thing as “the standard”.

[@thetaomegatheta]

@@JasperJanssen ' … did you watch the video?' Yes, I did. 'And no, there is no such thing as “the standard” Do you seriously not know about the non-one-sided limits?

[@wtmayhew]

This limit problem is a good illustration of why making even a rough sketch graph of the function in question can shed a lot of light. Using a graphing as a qualitative analytical tool is too often overlooked.

[@levaniandgiorgi2358]

While i do agree that graphs are amazingly helpful,i believe more complex problems would be better suited for them,idk.. to me, the answer felt glaringly obvious from the start.

[@wtmayhew]

@@levaniandgiorgi2358 I largely agree. I looked at the statement and pretty much saw the answer immediately, but then I have the advantage over freshman students of having done math for close to 60 years. I’ve encouraged students to not shy away from sketching Bode plots or pole/zero diagrams in the EE courses I’ve taught. It is handy to look at a problem with more than one method to avoid mistakes. The backup method doesn’t need to be precise, just accurate enough to confirm your thinking is on track.

[@ivanzonic]

No reason to waste time graphing something like this

[@heylolp9]

Graphs do what Graphs are supposed to do, give you a visual representation of the abstract equation It's helpful for people who are stronger visual learners to link the reasoning and the answer together

[@sankang9425]

Graphs are very powerful. It's really hard to believe calculus was invented without using them. People make fun of 'trivial' stuff like rolle's theorem, but good luck proving them without graphs.

[@henrikholst7490]

Note to self: "L'Hôpital's rule can only be applied in the case where direct substitution yields an indeterminate form, meaning 0/0 or ±∞/±∞. So if f and g are defined, L'Hôpital would be applicable only if the value of both f and g is 0. "

[@homebird4765]

I made the same mistake

[@vedantlearns7516]

same mistake here😢

[@Sanji-ip1vd]

Same and got answer one

[@vintovkasnipera]

If it's a school or university question, L'Hôpital's rule shouldn't be accepted as a valid proof to be honest

[@homebird4765]

@@vintovkasnipera Why's that?

[@cvkline]

Cady Heron would have figured this out… it’s how she won the athlete competition in “Mean Girls.”

[@StaticBlaster]

I love the movie reference.

[@alejrandom6592]

Hate that scene, the epifany moment makes no sense

[@Stags28]

I used to love asking my students on these if their denominator was positive zero or negative zero. The transition from initial confusion to a-ha was one of my favorite gems from teaching.

[@krishnannarayanan8819]

Sorry, I don't understand what positive and negative zero mean. Could you please explain?

[@BlueGamingRage]

​@@krishnannarayanan8819shorthand for "approaches zero from the positive direction" and negative directing, respectively

[@l.w.paradis2108]

​@@krishnannarayanan8819 A shorthand way of saying to approach 0 from x < 0, or x > 0.

[@Keneo1]

@@krishnannarayanan8819it means 0 or -0

[@morijin5568]

@@krishnannarayanan8819 you could assume some number "h" which is a very small positive number. positive 0 means 0+h and negative 0 means 0-h . basically 0+ and 0- are approaching 0 from right and left sides respectively.

[@joaooliveirarocha]

I just woke up and this video was suggested. I haven't touched in limits for almost a decade so my thought was "Ive forgetten all of this". I've opened the video,watched for 3 minutes and I could feel the knowledge coming back 😂 so weird

[@dscarmo]

Thats how most people say videos are teaching more than school In reality its just bringing stuff back.

[@Maximus.Decimus]

😂😂😂😂😂🤗🤗🤗

[@a-bison]

it feels like an old rusty gear inside your head has started to turn

[@doncappuchino6928]

I'm still worthy feeling 😂😂

[@terryendicott2939]

So I guess that your knowledge has limits.

[@contextinvideos]

never took a calculus class in my life but i still end up watching these videos

[@jamescollier3]

took multiple calc DQ and didn't really like it, but watch

[@muneebmuhamed43]

studying in class 10 but still watched cuz why not 😂

[@jim2376]

Admirable curiousity. 👍

[@dominicj7977]

​@@jamescollier3 I never really liked math as a whole in college. Then once I graduated, I started learning it in depth, on my own . Then I started loving it. Now it has been 7 years since I graduated and I still learn it

[@operator8014]

My calc class explained SOOOO MANY of the questions I had about things that didn't make sense from earlier classes. Can recommend.

[@houghwhite411]

I haven't been to school for at least 5 years, and now listening to your explanation I could understand better what I could not back then

[@johnelectric933]

Thank you. The limit pops up a lot in engineering, not just on paper but in actual physical or electrical functions. That said, in electronics we use graph paper as writing paper so doing a graph with 3 samples is quicker.

[@Lolwutdesu9000]

Are we not gonna talk about the amount of markers he's got stored in the background? I've seen entire schools have less than that lmao

[@chxrryery4188]

He just buys the whole stock 😭😭

[@TheBigGuy.]

my school doesnt have shit, or sistem has the students spend the whole year in a designated room and the teachers move from class to class, so the classroom becomes the responsibility of the students, so things like markers and chalk have to be bought by the students.

[@atklm1]

What is there to talk about their amount? Like guessing how many there are, or how much they cost and did he pay value added tax? Or something metaphysical, like maybe the markers were the friends we made along the way? 🤔😄

[@classified9668]

I’m assuming he buys them and resells to make a profit

[@bobtivnan]

Knowing how this function's graph behaves gives all of the intuition needed. Vertical asymptote at x=1, positive to the right, negative between 0 and 1. My students often dive into the calculus without thinking about the precalculus. Sure it can be done without the precalc, but the confidence gets a big boost when we think about the graph first.

[@No-cg9kj]

And that's how you get 0 points for the question on an exam. You're expected to do the calculus on a calculus exam.

[@bobtivnan]

@@No-cg9kj read more carefully

[@bramvanduijn8086]

Approaching a limit doesn't require a Y-axis, you're needlessly complicating the concept and conditioning them with a euclidian bias in their thinking about numbers.

[@iamcoolkinda]

@No-cg9kj On any calculus exam, this question would be a multiple-choice or short answer question where you’re either right or wrong, no partial credit. Graphing is sometimes way faster than doing the math. just visualizing the graph i solved this problem in probably 2-3 seconds

[@thetaomegatheta]

@@iamcoolkinda 'On any calculus exam, this question would be a multiple-choice or short answer question where you’re either right or wrong, no partial credit' Literally none of the math exams that I took at university had multiple-choice questions. You needed to actually demonstrate your knowledge of the topic, and, in the case of specific problems like that one, you had to present solutions.

[@phild8095]

This brings me back 50 years. Thanks. I recently talked with one of the guys who was also in that class.

[@frostshock13]

I haven't taken calculus in my life and was interested. Only to be completely distracted by the lifetime supply of expo markers.

[@its_elkku135]

I love this bite sized math content, feels like I'm getting a bit smarter every day :D

[@cmyk8964]

Finally! A problem on this channel I could solve on my own!

[@RavenMobile]

This channel is way out of my league 99% of the concepts he deals with... but I still come back to watch more, lol. I like how he explains things and how he writes on his whiteboard.

[@tombratcher6938]

BRP: "You can't just say this is undefined". It's undefined.

[@_Just_Another_Guy]

Btw, the graph shows that the two lines doesn't meet at a certain point (diverging on an asymptote) so that's what DNE literally means: there's no convergence at any particular point.

[@bartiii7617]

DNE actually means "does not exist" lol, a limit can still exist even if theres no convergence at any particular point, e.g. diverging to positive infinity/ negative infinity

[@bartiii7617]

maybe its TNCAAPP: "theres no covergence at any point"

[@hyperpsych6483]

@@bartiii7617 limits diverging to positive or negative infinity also do not exist by the delta epsilon definition, though most people just go with the "you know what i mean" equals sign

[@hydroarx]

​@@hyperpsych6483can't you use the epsilon-N/delta-M/N-M definitions for those limits?

[@alexatg1820]

@@hyperpsych6483I think it depends on the topology we’re working on, tho in common topology of ℝ we regard ±∞ as DNE, so I agree with you

[@HenrikMyrhaug]

Mathematicians: 0.999...=1 Also mathematicians: 0+ is not the same as 0

[@thetaomegatheta]

0+ is not an element of R at all. BPRP operating with those symbols that way irks me, as those aren't elements of any sets, let alone real numbers, and arithmetic operations are obviously not defined on them. The expression 'lim(f(x)) as x->0+ = A' simply means 'for every neighbourhood U(A) there exists S = intersection(U(0), {y | y is real and y>0}), such that for all x in S it is true that f(x) is in U(A)'.

[@kajus331]

@@thetaomegathetawhat

[@thetaomegatheta]

@@kajus331 Which parts do you not understand?

[@prycenewberg3976]

@@thetaomegatheta Everything after your username.

[@JohnSmith-pm3ew]

​@@thetaomegatheta They are trying to say they do not have the prerequisite knowledge to understand the mathematical expressions and theory behind what you wrote

[@benrex7775]

It's been a while since I did this type of stuff. Thanks for the refresher.

[@lengeschder]

I wish I had this channel when I was in calculus years ago. Thank you for a clear-cut explanation and getting me interested in math again.

[@ravenousstrike012]

I just literally advance self-studying Calculus 1 rn and this is my 1st video yt recommend it. I didn't know that we can also have exponential signs to determine if + or - infinity but i alr knew that it will be DNE bcuz of + & - infinity are not equal. Perfect timing! I can't wait for my next sem. You got a sub❤!

[@bprpcalculusbasics]

Glad to hear! Thank you!

[@zxcvbn089]

Im grade 7(ph) and i understand calculus :)))))

[@bananatassium7009]

brilliant explanation, these videos really make me feel like I'm getting a better grasp on calculus as someone who's never taken it but is passionate about math :)

[@jorwinwithcoffee]

Merry Christmas Bprp 🎉

[@bprpcalculusbasics]

Thank you! You too!

[@assetaden6662]

I still remember us being shocked when the teacher wrote positive and negative zero. We were perplexed, bamboozled even. Until he explained why and how.

[@0xinvestor]

Please HELP me. - at 4:53 how did you say that 1- was still positive as 0.99999 ? I need answert or else I wont be able to sleep and I have no one who can explain me like this. Please help.

[@thetaomegatheta]

Well, every neighbourhood of 1 in the real line contains elements that are less than 1 but are greater than 0. Specifically, 0.99999 is less than 1, but is greater than 0.

[@xinpingdonohoe3978]

Imagine a number very slightly lower than 1. Such as 0.9999. That's still greater than 0.

[@no_nuts0614]

1- is the number before 1

[@RajveerSingh-zl3mv]

1- or 1+ is used to denote that the number is neglegibaly lower than whichever number is used before the sogn therefore 1- lies btwn 0 and 1 which is positive . He said 0.999999... to show that its just a realy close number to 1

[@VegaOfficiaI]

I just got started learning calculus, and I've been having a hard time understanding horizontal and vertical asymototpes, not sure if it's because the way my teacher teaches or some other reason. But I after clicking and watching this interesting video, you just made something click, thank you!

[@thetaomegatheta]

'not sure if it's because the way my teacher teaches or some other reason' Given that you are just starting to learn calculus, and you mention being taught asymptotes, I'm willing to bet that you aren't actually being taught asymptotes and that your teacher is just not doing a good job.

[@iamtimsson]

your teaching style is comforting i still dont understand this one due to my lack of foundational knowledge, i think. still very glad to have your vids

[@RedGallardo]

The right usage of infinity is a gulp of fresh air.

[@userchrh]

I've found another method. We know that x/(x - 1) = ( (x - 1) + 1 )/(x - 1) = 1 + 1/(x - 1). So the limit equals 1 + lim_(x -> 1) { 1/(x - 1) }, or just 1 + lim_(x -> 0) { 1/x } which we know DNE.

[@colinjava8447]

When I took a course on it, lecturer said it doesn't exist (rather than its infinity), but on the video he's calling it infinity and -infinity, and for that reason the limit doesn't exist. I think I prefer saying it doesn't exist, but saying its infinity or -infinity gives you more insight into the shape of the graph I guess.

[@janskala22]

@@colinjava8447 The limit exists if and only if the right limit equals the left limit. If left limit is different from right limit (in a given point), the limit does not exist. The limit is not "either inf or -inf", it just "does not exist".

[@colinjava8447]

@@janskala22 I know, that's how I knew in 2 seconds that it doesn't exist (cause left =/= right). My point was in the video he writes infinity, when like you said it just doesn't exist. I think he knows that probably but does it for convenience.

[@janskala22]

@@colinjava8447 He only writes infinity on the right limit when it holds. He writes -infinity on the left limit where it holds. He does not write any definitive answer to the whole limit until he is sure it's DNE.

[@colinjava8447]

@@janskala22 I know, I saw the video too.

[@matthewlloyd3255]

Almost all limits can be evaluated by doing a thought experiment of "what happens if I move just ever so slightly to the left/right/both sides of it" - and then playing it out in your head.

[@ronaldjensen2948]

5:38 look at the asymptote on that mother function... Don't you have that t-shirt?

[@1luffiz]

what is "mother function" ?

[@quirky.science]

Mother function 😂

[@txikitofandango]

Bunch of ways you could do it. I like the old adding and subtracting 1 in the numerator. Then you're evaluating 1/(x-1) as x approaches 1 from left and right. Alternative: do a polynomial long division on this rational function. Then the answer is easier to see. Another alternative: divide numerator and denominator by x. Then you're evaluating 1/(1-1/x) as x approaches 1. Left and right hand limits are clearly different

[@wills4104]

I was taught that, if direct substitution results in A/B, where A and B are nonzero, that’s the limit. If you’re given a limit that is A/0, the limit DNE. If you’re given a limit that is 0/B, the limit is 0. If the limit is 0/0 it’s indeterminate. Methods like multiplying by conjugate, or L’Hopitals rule come in to play. So from first glance, you can instantly tell the limit DNE because the numerator is nonzero and the denominator is 0 when direct substitution is applied.

[@lugia8888]

You can have a limit equal to positive or negative infinity. Also, aside from Hopital you can use Taylor series.

[@wills4104]

@@lugia8888 limit equal to positive or negative infinity is typically considered DNE though, right? Because it approaches different values from left and right.

[@PixelVoyager777]

​@@wills4104 By definition, for a limit to exist in the first place, it must be a finite number. Both +∞ and -∞ aren't 'finite'. So when either the LHL or the RHL approaches either quantity, we say the limit doesn't exist.

[@jotch_7627]

​@@wills4104not quite. a limit at ±inf can absolutely exist, just not in the space of real numbers. it is extremely common to extend the space of real numbers with ±inf when working with limits. in this case the limit still doesnt exist in that space because it approaches both +inf and -inf from either side, but you can just as well define an extension to the space with only one infinity. its also trivial to make a function that approaches +inf (or -inf) from both sides with a form like A/0

[@inestabilidad]

I was taught to eliminate the same terms, so in this case I'd cancel both X and get 1/-1. My limit would be -1. Lol

[@Wedgievlogzclips]

We were taught that the general limit for that would not exist because one side goes to infinity and the other goes to negative infinity. You would have to do a directional limit

[@Yupppi]

Is there no fancy way like multiplying with x+1 or the usual trickery for limits I never really learned well enough to understand?

[@rando_prime123]

No. Try it yourself and see.

[@lool8421]

i feel like the answer is either positive or negative infinity, but it's not defined from which side are we approaching x, limits tend to have positive and negative zeroes that gives 2 possible outcomes for 1 limit so it just doesn't work

[@Umbra451]

Nice video! It’s been a while since I’ve done this. Since we did indeed get the conclusion that we would expect from inspection, could you give an example of a limit that looks DNE at a glance, but turns out not to be?

[@literallyjustayoutubecomme1591]

Well, that depends on how good your glances are, doesn’t it :)

[@Steve_Stowers]

@@literallyjustayoutubecomme1591 Agree. To beginning Calculus students, a limit often "looks DNE" as soon as they see that 0 in the denominator (even if the numerator also approaches 0).

[@jackbrax7808]

As someone else stated, it sort of depends on how good your “glance.” Is. If your very proficient with limits and calculus, you potentially could have know just by looking at the limit what the answer would be. But a great example in this case would be x/(x-1)^2. Having a square term in the denominator actually causes the limit to approach positive infinity from both the left AND the right. Therefore the limit actually approach’s infinity and therefore does exist!

[@Steve_Stowers]

@@jackbrax7808 Depends on what you mean by "exist." According to most basic Calculus books I'm familiar with, if the limit is ∞, the limit doesn't exist-you're just being more specific about how/why it doesn't exist.

[@jackbrax7808]

@@Steve_Stowers I just double checked my definitions and turns out your right. It doesn’t exist but both sides tend to infinity. But due to infinity not being a number, it doesn’t exist. But you can say the limit tends to infinity.

[@KabilanIITDELHI]

0:34 Got absolutely shocked and embarrassed😅😂

[@franciscom.bodinho2219]

Every time i see a limit i just l'hosptal the thang and pray for the best 😭😭😭

[@samikshakahali2472]

bruhh 🤣🫂

[@nasdfigol]

I immediately thought about which direction when I saw x->1 and x-1

[@glumbortango7182]

This question gets a bit easier if you rewrite x/(x-1) as (x-1+1)/(x-1), which is 1 - 1/(x-1), which is a little more obvious in how it behaves as x -> 1. Fairly simple explanation though.

[@maxime_weill]

Meh

[@Galactic-MathWizard]

Shouldn't it be 1+1/(x-1)? Also this was my first thought too!

[@utsavthakur6879]

U can also take For right hand limit x=1+h where h-->>0 And for left hand limit x=1-h where h-->>0 For limit to exist, Right hand limit = Left Hand limit In this question RHL=+2, LHL=-2 therefore limit doesn't exist

[@thetaomegatheta]

'U can also take For right hand limit x=1+h where h-->>0 And for left hand limit x=1-h where h-->>0 ' No, you can't. You have to use h->0+ and h->0-. Otherwise, you are just looking at a limit as x->1 either way. 'For limit to exist, Right hand limit = Left Hand limit' Strictly speaking, that's not true. That depends on the topology of the space where we are looking for a limit in. 'In this question RHL=+2, LHL=-2 therefore limit doesn't exist' In the case in the video, with lim(x/(x-1)) as x->1, assuming that we are looking for a limit in the standard two-point compactification of the real line, the RHL is +infinity, and LHL is -infinity, as shown in the video.

[@Felinaro]

From the times, when I was a student, I remember three different intinities: "+∞", "-∞" and "∞". So we explicitly used sign, if the infinity had one, and not used if that was "just the infinity", when sign is unknown (or does not matter). Is this the case nowadays? You are never using "+∞" notation, always omitting "+" sign...

[@Mr.Not_Sure]

Same

[@thetaomegatheta]

The space that is assumed in the video is the standard extension of R with two points at infinity - +∞ and -∞. Unsigned ∞ does not exist in that space. I think it's a bad decision on the author's part to not explicitly state what space we are looking for a limit in, as in other extensions of R that limit does exist.

[@Ligatmarping]

Yeah, this is a pretty basic video that doesn't enter into that, but that's exactly how we treat in class (I teach at University of Buenos Aires). Most time basic calculus students mess with the + or - infty and I tell to just rite infty symbol since we just search for vertical asymptotes only. So we just care on when the function collapses. Although there are some teachers who put enphasis on the sign, I don't agree to go into that when the students are struggling to more basic things at that level.

[@4fgaming925]

Im thankful to my teacher for giving importance to the concept of approaching from left and right side otherwise I couldn't have solved it

[@mehulgamer2836]

I was not getting LHL = RHL so I knew the limit does not exist. Btw Merry Christmas bprp :D

[@BossDropbear]

lim x/(x-1) =lim 1+1/(x-1) x approach 1 from above function approaches +infinity x approach 1 from below function approaches -infinity

[@weo9473]

Everybody gangster until g(x)=0

[@juanmacedo8816]

-_-

[@edrodriguez5116]

Merry Christmas Mr. bprp!

[@jmz_50]

Good video as always

[@IK_Knight]

Using precalculus and algebra 2 techniques we can easily determine the asymptote of the function and later the limit x/(x-1) we can use zero product property on (x-1) which will give us a vertical asymptote of 1 so when approaching x->1 u will get both infinity and -infinity showing that our limit DNE

[@nirorit]

I personally prefer using two sequences to show that. X_n=1+-1/n, and then the functions turns to to 1+-n, and when you take its limit you get +-infinity. Feels more rigorous to me.

[@pinnaclerigde3056]

it is ±infinite depends on which side you take. - if approach from -inf to 1 + if approach inf to 1. You can easily check using scientific calculator. Type in the function and calculate wiith x= n±10^(-6)

[@thetaomegatheta]

'it is ±infinite depends on which side you take' lim(x/(x-1)) as x->1 considers points in the entire neighbourhood of 1. You are thinking of one-sided limits.

[@alextonev3017]

All you have to do for most of these questions is think “how will the graph look like” and it makes it relatively easy

[@clmasse]

It's normal, there is no limit. Ask the good question: what is the + or the - limit? The problem is often the nut behind the whiteboard.

[@marvinliraDE]

If you have a function 'f' which is defined on a subset 'M' of real numbers and you have some real number 'y', then the left-side limit 'lim_{x->y-} f(x)' is defined as the limit 'lim_{x->y} g(x)' where 'g' is the same as 'f' but restricted to the subset of 'M' containing only the numbers that are at most 'y'. The right-side limit is defined analogous.

[@marvinliraDE]

So visually speaking, you cut your function at the point 'y' into a left side and a right side and handle each side on its own.

[@OnurOzalp-personal]

how did u know i nut behind the whiteboard? also he answered those as + and - infinity already.

[@clmasse]

@@marvinliraDEThe point is, the limit of the function defined on ℝ\{1} doesn't exist. The problem is in the question (asking for something that doesn't exist,) not in the answer, for DNE is not the limit of the function.

[@dielaughing73]

​@@clmasse I'm not sure there's anything wrong with asking a mathematical question for which there is no defined answer. Would you feel better if the question was "what is the limit of as x approaches , if such limit exists"? Because I'd think the qualification is implied for students beyond the most rudimentary level of maths.

[@erik9420]

I mean... just consider values of x nearing 1 from both sides. So try x=1/2, x=3/4, etc... and x=3/2, x=5/4, etc... It's pretty easy to see that the limit from the left is negative infinity while the limit from the right is positive infinity.

[@theknightikins9397]

While I do like the idea of the plus on the zero meaning a number arbitrarily close to zero, for problems like this I always think of it a “positive” zero. It’s functionally the same and gets the same answer, I just find it easier to understand, if you divide a positive by a positive, you get a positive. Is the zero positive or negative, not really, but if anyone is having trouble understanding this, try thinking of it this way.

[@fioscotm]

Huh, that actually is a really nice way of thinking of it. Thanks for this!

[@tomekk.1889]

It might help you with limits but it's not functionally the same. It's worth learning what 0+ actually means and sticking to that it will help you later with series etc

[@perplexedon9834]

I dunno if its strictly valid, but my instinct is to transform it using fraction decomposition: x/(x-1)=1+1/(x-1) and substitution of t=x-1 to be: lim t→0 1+1/t Which is immediately obvious as undefined

[@r75shell]

Do you think a teacher or other person who check your work will be fine with 1+/0+ argumentation? I think better would be substitute y = (x - 1) so, we need to calc (y + 1) / y with y -> 0 then (y + 1)/y = 1 + 1/y, thus for y > 0 it (y + 1) / y > 1 / y, but 1/y -> infinity but for y < 0 it doesn't work. so I think just use some fact lim (x + C) = C + lim (x). idk.

[@dielaughing73]

It's how I was taught at uni. Best to check with your professor if you want to be sure. Don't forget you can always (and often should) add annotations in plain language explaining what you're doing and why. Then it doesn't really matter what notation you use, as long as it's clearly defined and consistently applied.

[@AryanRaj-fz7dd]

​@@dielaughing73 our professor also uses these notations what the hell is wrong with it

[@billmilligan7272]

This is how I was taught as well. If a teacher or other person who will check your work isn't fine with it, it's time to talk to their boss.

[@kazedcat]

Just replace 1+ with 1+ε and 1- with 1-ε.

[@anonymousf7byyj]

@@billmilligan7272thanks for your input Karen

[@KabilanIITDELHI]

Very thank you sir!...🙌❤️, I've now Learnt the Importance of Mathematical reasoning

[@andrewparker8636]

I personally think it's easier to approach these problems by changing the limiting value to 0. That way it's obvious what's +ve and -ve. In this case, we could change the limit to lim e->0 (1 + e) / e by substituting x for 1 + e (e is supposed to be epsilon here). Then you can work out lim e->+0 and e->-0 and it's a little easier (IMO).

[@Math342010]

I really like this explanation since this explanation shows us that we mathematician did not do math recklessly according to the writing only, but according to the meaning of the limit hidden in the math problem.

[@Harishkumarindianrailways]

Why L hopital rule is not used here

[@OK-ei7io]

We don’t have an indeterminate form.

[@YourNeighbourJack]

Because L’hôpital rule only works with 0/0 or infinity/infinity

[@Harishkumarindianrailways]

Understood

[@teelo12000]

Because the injury isn't bad enough to go to L'Hospital.

[@J-M784]

@@teelo12000 This isn’t ‘la Páris’! 😂😂😂😂😂😂😂😂😂😂😂😂

[@dropBigMi]

my first thought it should be the same as lim (x->0)1/x (substract 1 and add 1in the numerator, simplify to lim(x->1) 1 + 1/(x-1), 1 is a constant and don't really change anything here and finally lim(x->1) 1/(x-1) looks almost like lim (x->0) 1/x, which explanation, usually always given in a class rooms).

[@welcometotheshow5247]

I always loved problems like this, it always reminded me that when u set up a number line the distance btw what ever numbers you end up choosing is infinite and if you wanted to count every number btw that distance you would always be approaching a certain number and never really reaching it.

[@olz6928]

0:53. I believe this is not completely correct. It could be the case for example that from the left the limit is minus infinity and from the right the limit is infinity. In that case we the limit wouldn't exist and we definitely couldn't say it is minus infinity or infinity.

[@adrified9352]

DNE since f has a VA at x = 1 and f is odd

[@epikherolol8189]

Vertical asymptote?

[@thetaomegatheta]

If we just assume the standard extension of R with two points at infinity, then yes. If we don't, there is another fairly standard space where the limit does exist - the standard extension of R with one point at infinity.

[@xinpingdonohoe3978]

f is not odd at all.

[@joyneelrocks]

You can use graphical methods too: x = x - 1 + 1 ==> x/(x - 1) = 1 + 1/(x - 1). So y = x/(x - 1) is basically the 1/x graph shifted to right by 1 and up by 1 unit. As x -> 1, x/(x - 1) diverges. So limit does not exist if you know the 1/x graph well.

[@colinjava8447]

I guessed that it was undefined in 2 seconds, cause its essentially 1/x, and its a limit from both sides.

[@jorymil]

Let's say you just shifted the coordinate system to the left by one unit by substituting (x = x + 1). Then you have lim x + 1 / x as x approaches 0, which might be easier for some to see. As you say, essentially 1/x.

[@electrodynamicorb6548]

It’s been 16 years since I’ve taken any calculus but knew right away it was undefined or does not exist

[@bDe4d]

Man, my teacher has been hammering our class wuth this for months and bearly anyone understood even the priniples of "limit" and how you check for it. I'm just now trying to keep up with the material as we're quite past that and even had a small test (which I failed, miserably) and I'm going to definitely retake it soon as thanks to you I understand everything perfectly, even though English is not my first language haha Lots of love from Poland! Cheers!

[@thetaomegatheta]

'Man, my teacher has been hammering our class wuth this for months and bearly anyone understood even the priniples of "limit" and how you check for it' Let me guess, your teacher said that a limit is something that a function gets closer and closer to as its argument gets closer to some point? Yeah, I'd advice looking up an actual definition of a limit.

[@Crazy_Diamond_75]

If you've done the x/x limit before, it's super easy--graph looks the same just shifted 1 unit to the right due to the "-1" in the denominator.

[@uberless1]

I am absolutely certain that my Calculus teacher from 20ish years ago would have hated your 0+ notation…. She wanted derivative tests all the way. EDIT: To clarify, she would obviously have been fine with 0+ in the initial limit, but she wouldn't have liked 0+ as a result of partial computation. [Though it does seem intuitive as shown in this video.] She would have considered this an "invalid shortcut".

[@tomctutor]

f' (x) := lim (h->0) (f(x+h) - f(x))/h well that's how I was taught how to find the derivative, using First Principles as it were. However when they teach Calc1 now they miss out this and expect you to look the derivative up in a table, usually supplied with the exam, whats the point of even learning calc this way! 😟

[@bernhardbauer5301]

1/0 is not allowed. 1+ and 1- are not numbers. 1/x has a singularity at x=0. This singularity is shifted in his example.

[@StAngerNo1]

I don't know why this video got recommended to me. I would not have know the correct notation, but figuring out the answer is quite simple.

[@bprpcalculusbasics]

Is this a well-defined limit? Calculus question on Reddit r/askmath kzbin.info/www/bejne/jZKpdqSkhaytedE

[@xetothex]

It’s better if you draw it on a cartesian plane, makes a lot more sense. Also, Instead of saying 1+, i’ve been taught this means approaching from the right side of the x-axis, and 1- is approaching from the left side

[@melonenlord2723]

Of course class couldn't figure it out if no solution exists 😂

[@isaacbruner65]

There is a solution and the solution is that the limit does not exist.

[@omp199]

​@@isaacbruner65 To say that the limit does not exist is just another way of saying that there is no solution to the problem of finding the limit. The statement that a solution does not exist is not in itself a solution. If it were, then you could say that every equation has a solution, which makes a nonsense of the concept of a solution.

[@kazedcat]

​@@omp199it's different with limits. "The limit does not exist" is part of the set of possible solution with limits. Similar to NaN is a possible answer to a floating point operation even though NaN literally means Not a Number.

[@omp199]

@@kazedcat No. It's not "different with limits". A solution is a value or set of values that satisfy a given set of conditions. If the condition is that of being the limit of an expression, then the nonexistence of a limit implies the nonexistence of a solution. As for "Nan", you are bringing programming language conventions into a discussion of mathematics. A programming language might have a function that returns NaN in certain circumstances, but that has nothing to do with mathematics.

[@kazedcat]

@@omp199 Programming is mathematics. The Turing Machine is a mathematical object.

[@andybogart2503]

It’s helpful to me to graph it so you can see the two branches of the function diverging near the asymptote.

[@darcash1738]

Nice, this is exactly how I did it. Can you post more proofs for common derivatives using the limit definition of (f(x+h) - f(x))/h? I think it could be fun to do a whole series on that. I tried the polynomial one for myself, and was able to confirm that (x^n)’, using binomial theorem and being left with just the second term was nx^(n-1). I tried getting the derivative of e^x = e^x, but I couldn’t pull it off tho, wasn’t sure how to bring out the h 😂

[@Syndicalism]

exp(x) is factored out of the limit. The remaining limit is [exp(h)-1]/h which evaluates to 1.

[@darcash1738]

@@Syndicalism nice! I looked at the standard way I guess you could call it for evaluating the last part, where you say that some variable, eg k = the top part, so it becomes k->0 k/ln(k+1). Bringing the k up top to the bottom w reciprocal, and then log power rule it becomes 1/ln(e) = 1. The main part that was sort of unexpected for me was the start, setting the top to a variable. How might we stumble upon this-just trying it out bc it’s limit approaches 0 as well? Also do you think that mathematicians found out the derivatives first and then tasked themselves with proving them?

[@Anamk001]

The amount lf markers in the back 💀💀💀, my whole school has less than that

[@lucaspanto9650]

Looks like it just tends to ∞

[@isaacbruner65]

That would imply that it tends to positive infinity which is obviously not the case.

[@lucaspanto9650]

@@isaacbruner65 🤓

[@thetaomegatheta]

@@isaacbruner65 No, it would not. There are multiple extensions of R, one of which is assumed by BPRP in the video and has two points at infinity, and another one has only one point at infinity, which we can call 'unsigned infinity' for clarity's sake. In the case of the latter one, the person you responded to is absolutely correct, and it is a bad thing that BPRP did not explicitly bring up the matter of the space in which we are supposed to look for a limit.

[@aarusharya5658]

@@lucaspanto9650 Your dumbass said the limit tends to infinity. I doubt you're in a position to use that emoji.

[@carp3tbomb3r99]

We always learned to take the limit approaching form the left, the right and then combine them

[@epikherolol8189]

I figured out it DNE in 5s lol

[@General12th]

Do you want a sticker?

[@Sukunut]

@@General12th lol

[@derekyu4430]

in my opinion, this question is the best way to tell us why concepts are important.

[@vmycode5142]

very proud of myself for figuring this out before you told us. :D

[@luisrocha26]

Happy not to find an "obvious solution" like Infinity or so. After 11 years of my first calculus class, I'm careful enough not to find anything trivial hahaha

[@winners-r4z]

but if we use L Hospital rule and differentiate the numerator and denominator then we have 1/1-0 which equals 1 so by that method the limit should exist and should be equal to one

[@thetaomegatheta]

'but if we use L Hospital rule' We get nothing, as L'Hopital (has nothing to do with hospitals) requires either of the indeterminate forms 0/0 and inf/inf. 'and differentiate the numerator and denominator then we have 1/1-0 which equals 1' And that is obviously incorrect, as even in the interval (1/2, 3/2) (which is a neighbourhood of 1) we have the infimum of |f(x)-1| = 2 for x in that interval and f(x) = x/(x-1), meaning that lim(f(x)) as x->1 cannot be 1.

[@winners-r4z]

Dangg, thanks a lot dude@@thetaomegatheta just after reading that first line of your comment i was like " ahh shit , i forgot bout that" XD

[@OverlordSpirit]

How the hell his whole class wasnt able to solve it, it's literally the basic LHL=RHL

[@first-namelast-name]

You can also answer lim_x→1[x/(x-1)]=∞ without precising the sign, and it means that there is an asymptote at x=1 in this case, without telling from where the function approaches it. Be careful not to mistake it for lim=+∞, tho, this one would mean that the limit is defined. This is also why I would recommend to always say +∞ instead of just ∞ when the value is positive.

[@xinpingdonohoe3978]

Like square roots, common practice has people being sloppy. They don't do much to distinguish, for example, positive infinity from unsigned infinity, or the multivalued square root from a single branch (generally the principal one).

[@ChilltheImpaler]

Negative infinity makes sense to me only needed to put in a couple of values to work it out. Been over 10 years since I've touched limits

[@Bogosbinted12]

It’s a two sided limit

[@hellohabibi1]

Or just x/(x-1) = 1+1/(x-1) so it's 1+ (lim x-> 0 1/x) which is dne

[@somebodyxd069]

took me like 3 seconds but my lack of trust in myself and curiosity kept me here lol

[@GoatTheGoat]

At 5:43 the denominator should be written as -0(plus). It is a negative number, with a magnitude just larger than zero.

[@alessandrogiglio5712]

Just a question, the given limit wouldn’t express the fact that the function, when approaching 1 from the negative side, goes to -infinity while approaching 1 prof positive side goes to +infinity?

[@carultch]

Yes. This is why the limit doesn't exist, because you get conflicting answers, depending on how you approach it. In order for the limit to exist, all possible approaches have to yield the same result. For 1/x^2 when limited to the real numbers, both possible approaches yield the same result. But, if you account for complex number approaches, you get conflicting answers, and the limit doesn't exist.

[@angelmatematico45]

If the limit tends to infinity, it does not exist, because infinity is a concept, not a number. What infinity means in this context is that as we get closer to the limit, the value is always greater.

[@gsGawade]

Literally took a second to solve

[@ИванВасильев-б9ъ]

any infinity limit DNE. +inf, -inf and inf are just handy concepts to describe certain cases. Sayin that some inf limit does exist because it has a sign and other does not is just word gymnastics

[@christophersoo]

everytime you have a limit question, divide both the numerator and denominator by x^n

[@carultch]

That works if you are looking at the limit as x approaches infinity, or negative infinity. You just compare the highest ordered powers of x, and ignore all the other terms. If they are both the same power, the limit as x approaches either infinity is the ratio of coefficients. A higher power on top, means it approaches one of the two infinities, depending on the sign of the coefficient and the net power on the ratio of x. A higher power on bottom, means it approaches zero. For limits at finite values of x, it's the net multiplicity of poles and zeros at that point that matters. I.e. number of poles minus number of coinciding zeros. More poles than zeros means the limit doesn't exist; more zeros than poles, means the limit is zero.

[@Mr.man22405]

I feel that it would unsigned infinity because it is just both neg and pos infinity at the same time. And when we look at one of those graphs that have a sphere (I forget what is called) pos and neg infinity are at different sides of the axis of the sphere and 1/0 would just kinda be directly in the middle of the axis and that would be an unsigned infinity. Edit: I could just be yapping though (I’m only taking algebra 1 rn)

[@thetaomegatheta]

'I feel that it would unsigned infinity because it is just both neg and pos infinity at the same time' That's not how this works. BPRP is looking for a limit in the extended real numbers, which doesn't include 'unsigned infinity' as a point. Meanwhile, the other commonly-used compactification of R - Aleksandrov compactification - does not include the 'signed' points at infinity. You could look for a limit in the space that has an 'intuitive' topology and includes all of those points at infinity, but you would end up with a non-Hausdorff space, which would introduce some issues and pecularities. I recommend you study a bit of general topology to get a deeper understanding of this stuff.

[@Mr.man22405]

@@thetaomegatheta ok

[@gary9793]

Why? substitute s=x-1 immediately yields x/(x-1) = s/s+1/s as s->0+ positive inf, s->0- negative inf. Therefore undefined