Inequality Mathematical Induction Proof: 2^n greater than n^2

In this video I give a proof by induction to show that 2^n is greater than n^2. Proofs with inequalities and induction take a lot of effort to learn and are very confusing for people who are new to induction.
I really hope this video helps someone!

[@leecoates]

I love this channel. Im an aspiring mathematician and frequently encounter overwhelming self-doubt about my ability. But when you explain something and reassure the audience that you struggled also, it is uplifting to know that it is not just me struggling with seemingly easy concepts. Seriously, thank you so much for this.

[@Dottedshine]

I just had an assignment due today, containing this exact problem. This is a very clear way of explaining it!

[@TheMathSorcerer]

Oh wow what a coincidence!!

[@ChandanKSwain]

@Kathleen McKenzie yes there mentioned that n>4, but he put k=4 , in the middle equation.....

[@bigmansanister8716]

@@ChandanKSwain yea, that confused me as well

[@leonbehrndt2611]

@@TheMathSorcerer Same haha

[@pranjalsrivastava3343]

he discovered gravity xD

[@mathnerdatsdsu6149]

For clarification, I know I am very late to responding to this video, however, when you use k=4 you must be sure that the inductive hypothesis hold for that value of k. If you plug 4 into inductive hyp it actually fails to be true. You must use a value for k that you know the inductive hyp holds true for. In this case it would need to be k=5.

[@xreiiyoox]

yes exactly, that's the part i was confused at to why he put k= 4 when k is bigger than 4, your comment clarified me thanks

[@ayeyukhine466]

@@xreiiyoox I think he puts 4 because of < before k^2.

[@jimpim6454]

What are you talking about its an inequality he didnt 'plug in k=4' he replaced it! I e he threw it in the bin and replaced it with something we know for a fact is smaller than k . Since k is bigger than 4 replacing k with 4 forces an inequality it is him reshaping it so it ends up looking like the conclusion.

[@amacommey192]

You amazed me. I just came from Eddie woo and others for this question now YOU! It’s like you understand the most basic intuition needed to solve it and you did it in so little steps. your solution is gold man. even for the factorial question. THANK YOU SO MUCH

[@TheMathSorcerer]

Aww thank you!!

[@raymondphiri8587]

So true!!!! You are good🙏🏽🙏🏽

[@Hello_am_Mr_Jello]

Same, just came from Eddie

[@Qubit313]

same

[@Chrisymcmb]

Thank you so much! You really inspire to continue on with school through this math stuff. Sometimes I feel very unmotivated with math because I'll try and I'll try, and when I get it, it's awesome. Plus it's something I genuinely enjoy, so it sucks sometimes when something is just not clicking. Anyhow, I've been watching some of your videos apart from the instructional math ones and they're definitely inspirational, thanks!

[@eguineldo]

I've been struggling a great deal in my proofs class and was self-conscious about my ability to think critically because of it. After watching this, not only do I understand the concept, I feel that I have a greater understanding of how a proof proves its claim. Thank you so much for this video, it has helped immensely!!!

[@aminakhan1195]

Everything's so clear now that I wanna cry oml! THANK YOU!

[@TheMathSorcerer]

You're welcome!!

[@syremusic_]

This is an extremely good video because you stumbled (or pretended to :) ) a couple times and talked us through how you figured it out. That’s super helpful. Thank you.

[@TheMathSorcerer]

😄

[@TheMathSorcerer]

Thx😄

[@nkeuphonium]

I appreciate the intuitive approach you take - so much of PMI instruction involves chaotic jumps in reasoning that are hard for listeners to follow and seemingly impossible to intuit ("how did you know to do that?"), so your decision to work with a problem you didn't already know is a great help. :) I was able to get this one a different way, but I had to use a pretty ugly derivative in the middle; your method is much more elegant.

[@akindantagonist8708]

Love your channel. So laid back and cool. Helping me so much with my math major. Tysm!

[@TheMathSorcerer]

You are so welcome!

[@EsotericArnold]

Aww man, this was beautiful, you were down to earth and showed very clearly all the things I missed from too many conversations with my professor. I actually have a good idea now, of how I should think when doing these inequality proofs. Absolutely amazing. Thank you!!

[@doctorduggo1873]

Wow, thank you so much! Excellently explained and easy to understand after you think about it a bit.

[@hypnogri5457]

How can you replace the k by 4 if it has to be >4?

[@DodiHD]

he messed up there but k^2 + 2k + 10 is still > k^2 + 2k + 1.

[@marangelitorres4515]

@@DodiHD I don't think he messed up. He is not saying k=4, the inequality says > k^2+kk, so whatever is on the left side is greater than this. So using 4, we are saying it will be greater than the value obtained when substituting 4.

[@CallBlofD]

How you know for sure that it will be greater from the value obtained after substituting with 4?

[@isittrueisitnot3303]

I think it goes n>=4 because we had the exact same task like this it was only n>=5 so it's probably a mistake he didn't notice but still correct..

[@nyashadzashegava9568]

'CAUSE K》4.

[@shadysama3143]

How did you replace k with 4 when you're assuming for some k>4? Aren't you supposed to replace k with a number greater than 4 because its not k >= 4?

[@manojtomer0709]

As the problem says n > 4, should we not use 5 instead of 4 in the inductive step? At 6:42

[@757Media]

That was so cool. I am barely starting my classes for my degree and I understood nothing, but it was very cool seeing you work out the problem. Some day I’ll get it.

[@okohsamuel314]

vashTX ... U said "some day I'll get it" ... meaning, u still haven't gotten it.

[@user-fe6mx9ui8h]

My prof had 1hr and 30 mins to explain this topic and you nailed it within 9 mins. I understood your explanation better than my prof.

[@TheMathSorcerer]

Thx, this is a hard topic to explain! I remember learning this myself and just not getting it. I ended up giving up and only understood it a year later when I looked at it again.

[@Amantheparadise]

@@TheMathSorcerer looking again,is also a mathematical step ,it works

[@bubbaganoosh]

This was extremely helpful after weeks of struggling. Thank you very much. :D

[@TheMathSorcerer]

Excellent!

[@doctorscalling9479]

My teacher tried to prove instead that the difference between the inequalities is bigger than zero, I myself find that much more confusing so when I saw this, I was able to solve any problem of inequalities, thanks alot you are going to save my grades.

[@israelchavez7544]

I can't say how helpful this was. I will now be ready for class tommorow. THANKS!

[@Andwoo101]

This was amazing. Thank you so much. This is the 8th place I visited trying to find an intuitive explanation.

[@TheMathSorcerer]

Excellent, glad I helped😃

[@flidoofficial1848]

Thank you so much for doing this video, I’ve been trying to understand this for weeks

[@giovannicalafiore7790]

What a smooth proof and explanation, simply wonderful, i love induction as I loved this video!!

[@raghad5270]

genius! I don't know how to thank you, I was in a trouble and this video saved me, a lot of thanks again..

[@TheMathSorcerer]

You are welcome😃

[@moodymonstrosity1423]

If k>4 why do we put 4????

[@aminakhan1195]

KZbin SHOULD OPEN A SCHOOL FOR ALL THE KZbin TEACHERS THAT TEACH BETTER THAN SCHOOL TEACHERS. PERIOD.

[@mr.knowitall5019]

@SteveEarl Watt?

[@beri4138]

@Eyosias Tewodros Are you a robot?

[@schizoframia4874]

My ears hurt 🩸

[@melbournebaldove6189]

Thank you very much for your videos. Do you have a good book that really tackles inequalities to have a mastery in them?

[@atuly7689]

never saw a more enthusiastic teacher on youtube 👍

[@omranone1058]

I really liked this method, thank you for your effort .

[@rdguezc]

great video!! thanks for sharing your knowledge. I have a question related to the substitution done in the minute 5:00 of the video. You said that "..you allow to do that (the substitution of 2^k by k^2) in math" and change the '=' symbol by '>'. I really want to understand how this substitution is possible and I want to know if you could provide us with any reference or material in which we could go deeper into this subject. Thanks in advance and again, thanks for sharing.

[@jimpim6454]

Its because he is replacing 2^k with something he knows is smaller than it namely k^2 so obviously the equality does not hold anymore so he must write the greater than symbol.

[@kuldeepsharma-oc5fo]

thanks sir for the solution I was stuck on this question from last 3-4 hours. great help.from india.

[@TheMathSorcerer]

👍

[@clawjet6069]

Best explanation I heard. First I thought this problem and my assignment from my pre-cal class was the same but it was actually the opposite " Prove n^2 > 2^n for n >= 5 " After watching the vid, I knew that the statement is already false so how do I show that the statement above is false using The Mathematical Induction?

[@rhodanyangu168]

This is amazing, I was given the first question to work out. Thanks 😍

[@Art-fn7ns]

Fantastic! Around 5:00 you managed to easily explain what our professor has been failing to...

[@TheMathSorcerer]

👍

[@jeroen3648]

Thank you for this tutorial, I was struck with this question, and your video helped me understand. :>

[@sanaalshaar5406]

Thank you. I was stuck on the '2^1 x 2^k' for a really, really long time. Induction is tough, and I am really overwhelmed but this video has helped me feel better.

[@creamypeanutbutter6326]

Hi may i ask what property or theorem you used when you replaced 2^k to k^2?

[@musiccd1896]

Great video keep them coming. I remember i had the same assignament. Proof was for n>=3 in my case.

[@keis7153]

First of all, thanks for the video everything is more clear now. Today I had my first exam at college and I had to proof that if A is a countable set then so is A^n by induction. Can you make a video of that?

[@TheMathSorcerer]

Will try thank you for the idea!!

[@mariamihab9542]

I don't understand why we replace K with 4 we have K is bigger than for not equal , so I don't get this point

[@ericsabacan2801]

Hi there, sir. I find your explanation very clear. I have a project in school, may I use this video to help students learn induction proof. Thanks for your help.

[@nathanyao3525]

Thank you so much. After I see the solution to a proof question that I don't know how to do, I'm always wondering to myself, "how the heck was I supposed to know to do that?" Do you have any tips?

[@tauceti8341]

2:50 /3:20 /4:47 When dinosaurs roamed the planet xDDDD I love the humility. These are starting to click for me and it's exciting to mess with algebra like this

[@katashi1995]

Thank you for your video. K have a question... why does the 8 becomes 1 in the last part?

[@callumatkinson3143]

Thanks for the vid I've been struggling with this for ages. I'm a bit confused about where you substituted in 4 for k. How does that work like would it still cover all the values bigger than 4?

[@TheMathSorcerer]

I used to struggle with your question also, tons of people do. The simple answer is that it's because k >= 4, so you can make that substitution. For example say k >= 4. And say you have 3k + p then you can write 3k + p >= 3*4 + p = 12 + p that's allowed:) You could work it out the long way. We have k >= 4, so 3k >= 12, so 3k + p >= 12 + p but nobody does that, because it's too much work. So in general, we just substitute as above.

[@CallBlofD]

Thank you for your help! Can you explain why k>=4 instead of k>4, because at start it define as k>4, how you change it to also be equal, or on what you depend when you say it. Thank you!

[@Chrisymcmb]

@@CallBlofD I was also wondering about this. The problem states that k>4, not k>=4, so that is why I was wondering how the k could be substituted by 4

[@ashimbasnet6568]

how do you use the same method for 4^n > n^3 for all N . I try to open it up like that and got stuck at 4^(k+1)>= k^3 +3K^2 .k

[@sofiachalkias5767]

A saving grace for discrete math this semester :)) Sets theory proofs and now I found out you do induction too, LETS GOOO!!! I was wondering since we were working with k > 4 how you were able to substitute k = 4 into the equation. Because of the I.H it is totally plausible to do this but it would have to be k >= 4. Even for k>=4 this should work right? I assumed since k > 4 that we were only allowed to plug in 5 or greater for k since our I.H is greater than 4 not equal to it. Thank you!

[@sebastianohajda411]

I think because for the rule k>4, if we substitute k=4 in the LHS equation, then we know the LHS will be bigger than the substituted version of it because of the rule k>4. I think you can also you k=5, but then you have to use >= sign, since LHS can be bigger or equal to k=5 substituted version of it

[@inquisitive871]

I see other induction inequality videos that show a different method. I find this method much more comprehensive. Would it work for all induction inequality proofs?

[@TheMathSorcerer]

Yes, absolutely, the ideas are the SAME for most of these!! thank you glad it was helpful, induction inequality is so hard to learn!!

[@chayajayamanna8823]

Excellent way of explaining. Night before the submission date. Thank you Sir

[@TheMathSorcerer]

You are welcome!

[@SpartaSpartan117]

So my instinct would be to pivot once you get to the “>k^2+k^2" to proving that k^2>2k+1 for all k>3. I wonder if there is any downside to that method; specifically in how that approach of basing the proof off of another lemma may fail when it is a more difficult problem and perhaps the dependency I need is harder to prove. Any thoughts?

[@TheMathSorcerer]

that works but, it's also more work;) but yeah that could work!

[@tonyhaddad1394]

Read my comment its easy i just proove it

[@jesuisravi]

Good. I've been in need of just this information.

[@TheMathSorcerer]

Glad it was helpful!

[@ujopozera6673]

Excellent tutorial indeed!

[@remlatzargonix1329]

Thanks for doing this! Cheers!

[@TheMathSorcerer]

You are welcome!

[@joonlokyap8392]

Thank you for your help bro. You're awesome 😎

[@imnotblackpinkfanimtheirai5300]

wow, you made this problem much easier. thanks

[@EastBurningRed]

Technically this is true for the open interval (4, infinity), so you need a more generalized induction that utilizes the well ordering relation.

[@danielgutter4354]

thank you very much. This helped me a lot :)

[@TheMathSorcerer]

You're welcome!

[@kopilahmed1887]

Here something i don't understand , here a condition that n>4 . How to you put 2×4 replacing by 4k?

[@someonewhoisgreek6186]

I am a bit confused. You replaced a k with 4 (I assume because that is the lowest value it can take ). Shouldn't the domain be k greater or equal to 4 in order to use four in the proof? It works with 5 as well, I am just curious as to whether this is a simple mistake or if I don't understand something. Can someone help?

[@gunarajregmi6727]

I am also confused on it . You can't use 4 . We have to start with 5

[@ibghxr]

I think he made a mistake, it was 5 imo.

[@abdoulkarenzo3138]

U are the first to teach very well me math induction thx a lot my broyher

[@TheMathSorcerer]

You are most welcome!

[@luuu_na35]

7:31 "Boom" the moment of enlightenment.

[@zerkyl]

Where did the 2^1 gone to?

[@haleematariq4184]

Good sort of information you delivered to the viewers.

[@TheMathSorcerer]

Thx

[@ANDREADELLAMAGGIORA]

Great video! The only thing I did not understand in the demonstration is why did you replace k with 4? if the hypothesis says it must be > 4 then shouldn't k be replaced with 5? Thanks a lot.

[@matko8038]

If you plug in k=5, the inequality will not hold. We want k^2+k*k to be greater than k^2+X*k. Our original assumption is that k>4 so we have to use some X that is less than k. k^2+k*k > k^2+X*k --> X4 we can use X=4.

[@vasuhardeo1418]

very cool to split it up, and yeh is a great look at proofs

[@michaeltheisen]

"when I was learning this stuff thousands of years ago..." the stories are true. he is a sorcerer......

[@nyashachikomwe8255]

😂😂😂I laughed hard, oh boy🤣

[@schizoframia4874]

I applied an inductive hypothesis for the original induction hypothesis and it seemed to work better

[@nelsonthekinger]

Good job man

[@upliftingspirit6873]

So we know that k > 4 is true in the hypothesis step. In the induction step, since n = k + 1, isn't it : n > 4 => k + 1 > 4 => k > 3 ?

[@maxamedaxmedn6380]

Oh thanks I think k>3 makes sense Because i was a hard time understanding why he used k=4 In the induction step and at the same time he says k>4

[@vincent2831]

at 6:24 we are claiming that (k^2) + (k^2) > (k^2) + (k*k), but shouldn't those be equal??

[@MelvinFung]

I thought K was large than 4, so shouldn't you substitute with 5 instead?

[@ChandanKSwain]

I don't know, how you placed 4 at the value of k, as it is mentioned that n >4....

[@shirleyeggenschwiler9482]

how can you replace 2^K with K^2?

[@israadaffaie5037]

Thanks for easy method to solution

[@sindbadthesailor4808]

The people want more induction proofs! Please do lots of them. (more tricky ones too)

[@TheMathSorcerer]

😄👍

[@alfonsovieyra9321]

How do we replace the 8 with 1? Why is that legal?

[@Qermaq]

The lower bound is like -0.7666647 ish. What is that?

[@brademerson8035]

I got completely lost when you suddenly replaced 2^k + 2^k with k^2 + k^2, I have no idea how or why that was done, and everything thereafter made no sense to me. I would really love it if someone could explain what happened to me, I re-watched the video like 4 times. And since when can we just start replacing variables with numbers of our choosing? I'm so lost.

[@yaloow]

Wait so how did the 8 turn into one

[@TechToppers]

How I did: Checking base case is easy... I proved another inequality before that: 2^m>2m+1 (for m>4) Make hypothesis and other stuff... To proof: 2^m+2^m>2(m+1)+1 (m>4) This reduces (by hypothesis) 2^m>2 (m>4) Works! Nice! Now to the main thing: Do hypothesis and base checking... To proof 2^n+2^n>(n+1)²=n²+2n+1 This reduces to(by hypothesis): 2^n>2n+1 Proved above! So, hence proved. I suppose. Is that right? I wrote it informally... Would do better in exam... I should have gone the other way round like first write 2^k>k², add inequality I proved and then proceed. You can spare me on KZbin right?? And tell if this is right... Please? Will you marks in exam? Or in spirit of math, is the idea correct?

[@farhansyed1038]

why can you replace k with 4?

[@jonathanwu5245]

How did he go from +8 to +1 at the end? I still don't follow? we're suppose to set it to equal to each other?

[@jonathanwu5245]

Okay, I think it makes slightly more sense since 8 is greater than one

[@oscardavidalarcon2673]

More please...

[@TheMathSorcerer]

Will do!

[@URKAEELIZACOLINPOWELL]

Sir,but it's k greater than 4,not k greater than Or equal to 4,might I know why did you put 4 as the value of k, kindly reply.

[@jonathan-xn4ev]

i did not get why i can replace k with 4, can someone explain to me?

[@swatijanmeda4942]

Exact question came in my exam.... Thanks a lot.

[@TheMathSorcerer]

Great 👍

[@oscarfc1993]

At 7:33 he has k^2 + 2k + 1. Shouldn't it be k^2 + 2k +1 + 7? If not how did he get rid of the 7?

[@mariamihab9542]

Why 7 ?

[@Changenow-ze2sl]

Thanks Jef!

[@MrTrollNerd]

Since the question doesn't explicitly mention only integer values of 'n', wouldn't it be more approapriate to solve it for rationals? Induction wouldn't be possible but maybe something involving the right hand limit of 4?

[@TheMathSorcerer]

it's supposed to be for integer values, oh hmm for noninteger values? I dunno I'd have to think about that one!!! Maybe what you say would work yes, not sure:) I think maybe subtracting it, and writing it like 2^x - x^2, then calling that f(x), and using some calculus, that might do it, maybe!!!

[@MrTrollNerd]

@@TheMathSorcerer I thought about it: both functions intersect at x=4, and the derivative of the 2^x term is always greater than that of the x^2 term for x>4. So it becomes trivial, I suppose. It probably doesn't make sense to make it more formal

[@riyatiwari1262]

Seems like maths also have exceptions!...well thankyouu so much it was understandable.

[@legend7890]

I took discrete math 1 year ago. I didn't understand mathematical induction. This semester I am taking theoretical CS and mathematical induction is needed so I am learning it again. This is the first time I understood a proof by Mathematical inducton.

[@ruantristancarlinsky3851]

Lol me too which University

[@arne8780]

At 4:56, I don't understand why we're "allowed" to replace 2k+2k with k^2+k^2.

[@GODCOR]

mehn.. i like the way you teach.. better than my lecturer.. lol

[@Sven1401]

This helped me a lot

[@TheMathSorcerer]

Awesome

[@sunsetsimp425]

I don't get how we are writing 4 if k>4, why not 5 like in the basic step 😩 someone please explain

[@Uwek212]

This deserves a big fat LIKE

[@TheMathSorcerer]

Haha thx

[@wellwell8025]

Wow thanks!

[@poppycock3954]

why do we replace the 8 with 1 near the end??

[@yeahno2466]

I'm also confused lol

[@jirajosue8800]

I'm quite confused, how is 8 the same as 1? can you please help me?

[@marangelitorres4515]

The conclusion that was reached was that 2^(k+1)>k^2+2k+8. If this is true, then it must me true that 2^(k+1)>k^2+2k+1. Eight is greater than 1, so if you have something that is greater by 8, it must be greater by 1. Hope this helps.

[@specialbiniyam544]

what an explanation! I really loved it sir, but, replacing "k" with 4 is not valid, as far as I'm concerned.