Interesting Angle Problem From Peru

[@zachtravel]

A simpler solution: From point A construct an angle CAE = x with E being the crossing point of AE and BC. Connect E with D. Angle CAE = Angle ECA = x --> Angle BEA = 2x. Angle BAE = 3x - x =2x. So in Triangle BAE, BA = BE. AB Parallel with DC --> Angle ABD = Angle BDC. BC = DC --> Angle BDC = Angle CBD. Thus, Angle ABD = Angle CBD. {AB = BE, Angle ABD = Angle CBD, BD = BD} --> Triangle ABD = Triangle DBE --> AD = DE, Angle BED = Angle BAD = 8x. Angle EDC = Angle BED - Angle ECD = 8x-4x=4x. Thus, in Triangle EDC, ED=EC. Then, we have AD = DE = EC = AE. Triangle ADE is an equilateral triangle. Angle EAD = 6x=60 degree, x = 10 degree.

[@Albertnetymk]

This is how it's supposed to be done.

[@stanwu1829]

that's what I was looking for. there should be solution without sins.

[@gibbogle]

Yes! To hell with trig!

[@mienzillaz]

Where is E?

[@SuperPassek]

Awesome!

[@rahulkangutkar6205]

One thing I learned from this channel is how much we limit ourselves... Solving such problems in school under specific chapters was much more easier as we already knew that in what direction we have to think and what formulas we have to use. But here, we have to think in a broader aspect. Thank you MindYourDecisions

[@noahtaul]

You can skip from 1:52 to 3:41 by using Law of Sines on triangles ABC and ADC. No need for any y's.

[@HIMANSHUKUMAR-vl7ob]

y is needed for equation.

[@mohanramanaramisetty]

Can u pls explain how ? I am unable to figure it

[@noahtaul]

@@mohanramanaramisetty easy, angle ABC is 180-4x so sin(ABC)=sin(180-4x)=sin(4x). So LOS says BC/AC=sin(3x)/sin(4x). But BC/AC=DC/AC=sin(5x)/sin(8x) by the same idea. Done

[@mohanramanaramisetty]

@@noahtaul Got it Thanks :)

[@emirakn4274]

we cant find the equation without y.

[@AshishSharma-yn5zi]

Apply Sine rule directly on given triangles ABC and ADC, since BC = CD and AC is common side. Reach equation at @3:40 without joining B and C, and going into unnecessary calculations with y. BC/AC=CD/AC gives sin3x/sin4x=sin5x/sin8x.

[@henri-leonlebesgue5471]

sin(180-x) = sin(x) is not a reflex for everyone but for sure it's easier when you remember it

[@soumyaghosh9637]

Can't it be solved without trigonometry....by applying geometry and sum of all angles 360 in a quadrilateral

[@TaismoFanBoy]

@@soumyaghosh9637 You'd be missing ADB, and you can find by contradiction that the two intersecting lines are NOT 90 degrees, so it's impossible to complete this problem by geometry alone. You'd have three variables and two equations, which isn't enough to solve it.

[@mitulkhanna6361]

@@TaismoFanBoy it is possible with some more construction

[@cpsof]

When Gougu is absent, al-Tusi and al-Kashi make their entrance.

[@canttankofficial4986]

Lol😂😂👍

[@mr.knight8967]

MATH QUESTION Algebra question Fun problem. kzbin.info/www/bejne/gZ-rYolpobuZiLs One time must watch

[@Zemlya01]

kzbin.info/www/bejne/i4LGiqqqad9-pbc

[@Raj-ix1wp]

Lol

[@MrRyanroberson1]

top upvoted comment is this lol

[@SohilRathi]

An easier alternative solution would be to directly solve for y in terms of x. Since BC = CD, we know that y = CBD = CDB. Then, we know that 2y + 3x + x = 180 so y = 90 - 2x. The idea to finish from there is to use Ratio Lemma (a well known competition math technique) which basically is an extended angle bisector theorem which is basically that the sides have a ratio of the sin of it's angle multiplied by the opposite side. After drawing side length BD, we get 4 different triangles. Using this fact that DBC = 90 - 2x and doing a bit more angle chasing, gives us all the angles in terms of x. Now, using Ratio Lemma on triangles ABC (well, for this case, since both angles are 90 - 2x we could just use angle bisector theorem itself) , BCD, CDA, and DAB, we get some trig equations and solving we get x = 10 degrees. AO/OC = AB/AC = AD sin(90 - 6x)/(DC sin(90 - 2x)) --> AB/AD = sin(90 - 6x)/sin(90 - 2x) BO/BD = BAsin(3x)/BDsin(5x) = sin(x)/sin(3x) --> AB/AD = sin(x)/sin(3x) * sin(5x)/sin(3x) Then, we equate these expressions. (This was simpler than what Presh did in my opinion) Then note that sin a = (e^(ia) - e^(-ia))/2i so we can solve using properties of arguments to get 10 degrees. In addition, I have a video on the basics of angle chasing.

[@alexanderhuang5511]

Oh wow, I never thought of that

[@jacksonwang7804]

Oh I think I now see why the sin y terms cancel, but definitely this is a little easier

[@kickwriteteach2313]

Yeah, this was my first intuition.

[@shubhendranathsingh9888]

This doesn't do anything lol

[@idk7016]

Doing that achieves nothing.

[@allgenre2332]

I understood everything in this video except AL TUSI LAW OF SINE

[@marksman2op]

Hey! This is called Law of Sine's in my country. It states that for a triangle ABC, let the sides opposite to angle A, B and C are a, b, c respectively. Then, (a / sin A) = (b / sin B) = (c / sin C). It can be proved using drawing a perpendicular and comparing the height of perpendicular in both newly made right angled triangles.

[@MrCoxmic]

Yeah, who is this Al Tusi person?

[@rezaasharifmohammad2340]

I think he was an Iranian scientist born in Toos on Feb. 24, 1201. Look at pukiwedia for "khajeh Nasireddin Toosi"

[@rezaasharifmohammad2340]

Oops, Wikipedia, sorry.

[@allgenre2332]

@@marksman2op thankyou

[@TomKaren94]

I wish Presh would start drawing these graphics to scale. There's no way AC and BD are perpendicular with the givens, yet that is how it is drawn.

[@kamalbandyopadhyay4681]

Simple solution through geometry: A triangle is 180 deg. Bottom triangle: 5x + 3x + 10x Top triangle: 3x + x + 14x Total of square: 360 deg T4: 36x = 360 X = 10 deg

[@mienzillaz]

How on earth you arrived at 10x and 14x. Angles ABC and ADC can only be expressed in terms of 180 minus angles with x. Putting that together gives 0=0 what makes x any value.

[@jackdog06]

Me and my Mum’s mathematical method: “Well let’s try 10 and see if that works, first..... oh it does..... I guess we win?” I am 19 years of age with an A level in further maths. But this was my method.

[@roberttelarket4934]

A unique trigonometry problem. These are the challenging sort that you never see in U.S. high schools today or in the past in the 1960s when I was high school student.

[@fernandodanielgomezcarnero6789]

It is the first time that I see an English-speaking person solve a mathematical problem of my country. I treasure it very much, thank you. There are more interesting peruvian exercises you gotta know, especially those of the national University of engineering of Lima. 🇵🇪🇵🇪🇵🇪

[@roberttelarket4934]

He is English-speaking but he(Presh) didn't solve it. Anoth Peruvian posed it on Twitter.

[@TheRealMVK]

Ah yes, solving geometry problems with trigonometry

[@virajagr]

Yeah, I was hoping for something out of the box. But it was just sine rule

[@rajivreddy149]

@@virajagr Hey Viraj, there is a nice geometric solution for this. I just think Presh is not that good at finding those solutions

[@virajagr]

@@rajivreddy149 probably. Infact I have sent him a few good problems but he didn't make a video on it. I guess he only makes it when it involves al tusi, al Kashi or gougu

[@marksman2op]

@@virajagr I am sure you are not the only one who sends problems. It's a huge channel.. approx 2M subs. How can work on every recommended problem?

[@virajagr]

@@marksman2op sure he has 2M subs. How come it's every time either so easy that everyone solves it from thumbnail only (as per comments) or a gougu thing?

[@artistyash6326]

In the triangle, x + 3x + B = 180 4x + B = 180 X + B = 180÷4 X+B = 45 (equation 1) In the triangle, 3x + B + x = 180 3x + 45 = 180 ( from equ. 1) 3x = 180 - 45 3x = 135 X = 135 ÷3 X = 45

[@omegamath5125]

Not bad!!! This is a rhombus! 1/sin(8x)=AD/sin(4x), 1/sin(5x)=AD/sin(3x) => 1/sin(8x)=sin(3x)/{sin(4x)sin(5x)} => 2cos(4x)sin(3x)=sin(5x) => sin(7x)-sin(x)=sin(5x) => sin(7x)-sin(5x)=sin(x) 2 cos(6x)sin(x)=sin(x) => cos(6x)=1/2 => x=10

[@kenkenplayer8920]

BTW It's not a rhombus, but I got the same approach as yours. There's no need to connect BD and introduce an angle y. Just apply law of sines on triangles ADC and ABC, and use the fact that sin(a)=sin(180 deg - a) as you have shown. Unfortunately many of the top liked replies have nothing to do with math but are just boring jokes.

[@omegamath5125]

@@kenkenplayer8920 You are correct! This is not a rhombus. When you extend the line BA, when you draw parallel to the line BC from D, You can get a rhombus! And one side of the rhombus can be set to 1. I dropped this explanation out. Sorry!

[@rajatsinha3863]

I think the ans will be 20°. Proof: ∆ABC ≈ ∆ADC (by SAS axiom of congruency) AC=AC {common} angle BAC=angle ACD {both 3x°} BC=DC {given} => By CPCTE, angle ABC and BCA(x) = angle ADC and CAD(5x) We know that x ≠ 5x => angle ADC = x & angle ABC = 5x => In ∆ ABC: 5x + 3x + x = 180° [ angle sum property of ∆] => 9x = 180° => x = 180°/9 = 20° You can check for the other ∆ADC also. And plz tell me if there is any mistake in my solution.....

[@sabanyaban822]

OMG! I tried to solve the problem without using pen and paper. Luckily after 30 seconds i gave up. Didn't expect the solution that long. Whereas the question seemed so easy.

[@SIGNUMMATHEMATICS]

Please tell me how

[@juanreyes1593]

kzbin.info/www/bejne/q3iwqXZsppmnnbs Only Geometry

[@sabanyaban822]

@@juanreyes1593 Your solution seems much easier, thank you

[@adityabose-bandyopadhyay7195]

What happened to his voice at 4:07 though

[@abhinabaroy6654]

চুপ বাড়া

[@GeoDetective]

And at 5:17. That happens when you edit the video, find out you said something wrong and record it again. I sometimes have that when I make my video's. During editing it's hard to keep your enthousiasm in your voice.

[@geoninja8971]

Hells bells! I was never going to solve that! :D

[@Bry10022]

How did you make this comment before this video was published?

[@allgenre2332]

@@Bry10022 omg how ?

[@allgenre2332]

@geoninja please tell i also want it know this trick

[@qtythandle]

Maybe he is using the law of sinx time

[@MathMadeEasy]

Bry10022 They are probably a patreon

[@CauchyIntegralFormula]

You can get the equation at 3:40 without all these extra constructions, just by looking at Law of Sines on the top and bottom triangles of the original problem. The angle at B has measure 180-4x, and the angle at D has measure 180-8x. So Law of Sines up top gives us BC/AC = sin(3x)/sin(180-4x) = sin(3x)/sin(4x). Law of Sines down below gives us CD/AC = sin(5x)/sin(180-8x) = sin(5x)/sin(8x). The side ratios are the same since BC = CD, so sin(3x)/sin(4x) = sin(5x)/sin(8x), which is equivalent to the equation at 3:40

[@henrykz2725]

No need to draw line BD. Just apply sine law: sin(3x) : BC = sin(180-4x) : AC and sin(5x) : CD = sin(180-8x) : AC. Given CD = BC, and sin(180-4x) = sin (4x) one gets sin(8x)*sin(3x) = sin(4x)* sin(5x)

[@cyriakharris4962]

I've never known about Al-tusi's law of sine so this way is much better for me. Thx

[@lonely6887]

Damn such simple question on first sight but yet you've taught me to not judge a fkin book by its cover

[@Super-Shafs]

I haven't learned about sin and cos yet, so me giving it a try is a lost cause

[@rishurg2655]

Yes Because there is a triangle formed

[@idk7016]

It IS solvable using mere euclidean geometry only, however I do not know the solution yet. He also included a link in the description to such a solution, but it has no explanations.

[@adrh2370]

@@idk7016 it is by using only triangle properties (Exterior angle and Sum of the angles)

[@idk7016]

@@adrh2370 Can you show me a solution?

[@sidharthpatra4751]

Ctrl W can u please show in detail how to do it through exterior angles properties. I tried using this property but still all the x terms are cancelling out

[@danilonascimentorj]

You can also do it geometrically with a very ellegant solution. You can construct a equilateral triangle BPC, where P lies on the extension of AD segment. Hence, CP=CD and then angle

[@michaelgolub2019]

Just it is to be proved that P belongs to AD extension...

[@danilonascimentorj]

@@michaelgolub2019 You do not need to prove P belongs to the extension of AD. You could even have point P inside segment AD and construct the equilateral triangle BPC inside of the two triangles. If it is not possible to construct an equilateral triangle in these conditions you would come to a contradiction. And the good thing is that you come to the conclusion that point P should be outside of segment AD after you do the same math.

[@GeomeTrazos]

What´s your opinion for this solution? kzbin.info/www/bejne/fWOooHqXhdhribc

[@mathetc.5807]

There is a very nice solution to this problem without any trigonometry. I will not give a full proof, but it simpler than trigonometry, took me less than an hour. |AB|+|AD|=|BC|. Look for isosceles triangles, and if we put a point E on BC, |BE|=|AD|, through isosceles triangles we get that ADE is equilateral, so that angle EAD equal to 6x = 60 degrees, x=10 degrees

[@GeomeTrazos]

See this solution Irina: kzbin.info/www/bejne/fWOooHqXhdhribc

[@kaanaslanturk6805]

You dont need to use trigonomety. If u find AB parllel to DC draw a line from B to D, by using z-rule ABD, CBD and BDC are same angle. Rest is up to u :) I relised it by extending AD and CB to make a triangle.

[@pkd2762]

I think Al Tusi is slowly taking the place of Gougu.

[@SnapquesterMage]

This reminds me how utterly frustrating and arbitrary trig identities and rules are. I remember it was just "memorize these dozens of identities" and I remember none of them today. I love me some calc, but goodness gracious do I remember hating trig.

[@fransschouten7997]

Forget the SINES crap.Draw the line BD which gives equal angles CBD and CDB (BC=CD). The angles 3x show that AB is parallel to CD and make angle ADC :π-8x (π=180 degs). Now every angle can be expressed in terms of π and x. The smallest angle is ADB : π/2-6x which should be >0 and thus x0 so that the solution is 0

[@ianrobinson8518]

I agree. I can’t understand why almost everyone else can’t see this but instead runs down convoluted (and usually error laden) arguments, often using superfluous trig. A moments reflection on the problem as specified shows it cannot be solved for a unique value of x.

[@deejay1964]

Maybe I am oversimplifying this problem, but would this be quicker: 1) Measure angle ADC (with a protractor) =100° 2) Total triangle angle of triangle ACDA = 180° 3) 180°-100º=80° 4) Therefore angles CAD=5x + ACD=3x = 80° 5) 80°/(5x+3x)=10° Result X=10° Perhaps it's just me or perhaps I am trigonometrically challenged.

[@flavioperuzzo1441]

Without too many constructions, it is easier to apply the Al-Tusi law of sines directly to the two triangles ACB and ADB : The angle in B is (pi - 4x), the angle in D is (pi - 8x), so: sin (3x)/sin (pi-4x) = BC/AC = DC/AC = sin (5x) / sin (pi-8x) which leads directly to the formula at 3.40 minute

[@khalifaalkhalifa6622]

If we try to verify our solution, x=10, then we get: 3x=30, 5x=50, y=70, the angle BDA=30. Everything is alright: triangle BCD is isosceles, AB parallel to CD, the sum of angles in the triangles and the quadrilateral are all fine. Now, if we take x=6 for example, then we get: 3x=18, 5x=30, y= 78, the angle BDA= 54. And all the properties mentioned in the previous sentence are satisfied. Is not that right? It looks to me that x=10 is not the only solution, but any x such that 0

[@fransschouten7997]

The angle BDA is : π/2-6x and the smallest thus x

[@GeomeTrazos]

kzbin.info/www/bejne/fWOooHqXhdhribc

[@blademuci]

came across this channel on youtube recommended. i tend to try and solve the problems myself without looking at the solution. for this one, i found x=15 through simple geometry and was surprised to find that x=10 is the answer, seeing as it doesn't work on my method. If you would like to find how i did it using simple trigonometry and the propriety of triangles, contact me back and i will probably make a video showing you

[@mienzillaz]

Making videos on how not to solve math problems is great idea for a YT channe :)l

[@nellymelendez2047]

wow, i've spent months trying to do it. Finally, a problem from my country, thank you!!!

[@fransschouten7997]

Triangle BCD is isosecles makingb angle CBD equal to CDB, also AB is parallel to CD because of the 3x angles making angle ADC : π-8x (π=180 dgs) so all angles can be expressed in terms of π and x Angle BDA is the smallest : π/2-6x en must be greater then 0 and thus x

[@alejandroenciso9650]

Nelly, mira aqui: kzbin.info/www/bejne/r6nTk5qkbLGLd7c&feature=share

[@GeomeTrazos]

Nelly, en este link podrás encontrar una bonita solución a este problema. kzbin.info/www/bejne/fWOooHqXhdhribc

[@fabioromucho157]

When you cancel the 2sin(x)cos(x)=sin(x) you needed to make sure that sin(x) is not 0, but is a angle from a triangle so there's no prob at all :)

[@davidash9170]

Once we have determined numerically that the correct solution looks like it is x=pi/18 (I prefer to deal in radians but in degrees this is x=10degrees), it is a bit easier to rigorously prove that x=pi/18 is the correct solution than to solve for it from first principles. For x=pi/18, 9x=pi/2 so 5x+4x=pi/2 and sin 5x = cos 4x. So sin 8x/sin 5x = 2 sin 4x cos 4x / cos 4x = 2 sin 4x and the rest is easy.

[@mithusaha1135]

In 2:30 the mentioned "AL TUSI'S LAW OF SINES" is aka "LAMI'S THEOREM".

[@cain259]

No lami's theorem is different I mean you are right but it is applicable in cases like Three forces acting on a point at angles A,BandC to each other But it's somewhat exactly same as law of sines in formula But both are different

[@MarieAnne.]

aka Law of Sines

[@cain259]

@@MarieAnne. yes But the first person is confused with law of sines as lami's theorem

[@shadrana1]

At 2:10, BC=CD=L say, (in this solution y is not required), Consider triangle ABC; Drop a perpendicular from B to intersect AC at M. Now consider triangle BMC; BM=BC sin(x)= BC cos(90-x) BM=L sin(x) =L cos(90-x)_...............(1) MC=L cos(x)=L sin(90-x).................(2) Angles of triangles BMC are therefore x,(90-x) and 90 degrees, Now,consider ABCD; Make CD the baseline and extend CD to NC to the left of the figure. Extend BC and AD to meet at F, Triangle FBCDA now exists. After a little elementary geometry we have an isosceles triangle FCD with sides FD=DC=L and FC=2L cos(4x) (FC is not required for this answer), Angle FCD=DFC=4x, Drop a perpendicular from F to NDC to intersect at N, We then have a right triangle FND with side FD=L, Angle ADN= 8x (angles FCD+DFC=8x), FN= L sin(8x)= L cos(90-8x)..................(3) ND= L cos(8x)=Lsin(90-8x)....................(4) Angles of triangle FND are therefore 8x,(90-8x) and 90 degrees, Triangles BMC and FDN are exactly the same with hypotenuse =L (90-x)=8x and (90-8x)=x Therefore, 90=9x and 90=9x >>>>>>>>>>>>>>>>>>x=10 degrees and that is our answer. This is the only real answer available since any other value for x would change the shape of ABCD.( 0

[@eylonshachmon6500]

Still don’t know who told you this is one of the best channels in KZbin..

[@richardkaris6146]

If you draw a line from B to D and give the angles names Y, W and Z where O is the intersection point so that angle ABO is W and OBC is Y and ADO is Z then you can solve. First you see W and Y must be equal (8X + Y + Z = 180 and 8X + W + Z = 180 are both true) which leaves 3 equations (after substituting W with Y): 12X + 3Y + Z = 360 8X + Y + Z = 180 4X + 2Y = 180 When you solve this, X = 10 (Y = 70, Z = 30) is the only solution I believe 'without' having to use trigonometry.

[@chuchlechuchle]

This is not correct. The system of equations has infinity many solutions, i.e. it is under-determined. For example X = 5, Y = 80, Z = 60 is also solution. You need to find one additional equation.

[@VK1710]

From mathematical proof we can get x equals to 50 as well by talking 6x =300 but as 8x becomes more than 360 it is neglected as it is an angle of quadrilateral.

[@gpetty91]

Another geometry problem ruined by advanced trigonometry.

[@saatvikagarwal6358]

You think this is advanced trigonometry??? You are in for a surprise

[@gpetty91]

@@saatvikagarwal6358 oooo, I'm shivering in my boots.

[@jeconiahjoelmichaelsiregar7917]

@@gpetty91 You call the law of sines, sums-as-products identities and some basic trig equations "advanced"? Clearly you haven't seen trigonometric calculus.

[@gpetty91]

@@jeconiahjoelmichaelsiregar7917 lol, I shouldn't have to explain myself to someone who clearly doesn't think beyond their knee jerk reaction.

[@bigbrain296]

@@gpetty91 clearly these aren't people who are familiar with euclidean geometry and its simplicity. I don't think we can help them much more than this.

[@jeremiahlyleseditor437]

Incredible This problem I never would have solved.

[@Dirkga]

ACD=180°=3x+5x+Yx in this case you see very simple that Yx=10x so if 180=10x+5x+3x then x=180/(10+3+5)=10 🤔😉 ABC=180°=3x+x+Zx in this case you see very simple that Zx=14x so if 180=14x+x+3x then x=180/(14+1+3)=10 in short a triangle always has a sum of angles that is equal to 180. if two angles are known then the third angle is a way of deduction. in this case two angles are a factor of x in which we can substract these two angles from 180. so as for ACD=180-(3x+5x)=10x and for ABC=180-(3x+x)=14x 😁😉

[@jackhandma1011]

Resorting to trigonometry means accepting defeat.

[@egillandersson1780]

Did someone find a pure geometrical solution ? Presh's one is very interesting by recalling us trigonometric formulas (like Simpson's one which are not often used) but it s quite complicated and not very elegant.

[@mrberry3483]

Yes. I've posted the solution here. kzbin.info/www/bejne/r6nTk5qkbLGLd7c

[@Zemlya01]

kzbin.info/www/bejne/i4LGiqqqad9-pbc .

[@39rama]

I posted this as a comment to Presh's video, but it gets buried with so many comments about Gougu etc. that are not really discussions about the problem itself. Anyway here it is. Let's call BC = CD = r and AB = d We can construct a parallelogram ABCE where E lies on CD such that CE = AB. Through angle chasing, we can determine the following: angle BAE = angle BCE = 4x angle ABC = angle AEC = 180-4x angle DAE = angle AED = 4x So, triangle ADE is isoceles with AD = DE = r-d Pick a point M in BC such that BM = d So, MC = BC - BM = r -d Draw a line connecting B and D to form a triangle BAD Connect M and D to form another triangle BMD With some more angle chasing, we can see that triangles BAD and BMD are congruent with MD = AD = r-d Connect points A and M to form triangle ABM Triangle ABM is isoceles with angle BAM = angle BMA = 2x It is given in the problem that angle BAC is 3x, so angle MAC = 3x - 2x = x It is also given in the problem that angle BCA = x, so angle MCA = x So, we have an isoceles triangle MAC with MA = MC = r-d We can now see that the triangle DAM is equilateral with side lengths (r-d) Since angle DAB is 8x and angle MAB is 2x, we can see that angle DAM is 6x Therefore 6x = 60, so x = 10

[@sutapadey5274]

@@39rama your solution is just like my solution with different letters. either you copied me or you are as creative as me

[@39rama]

@@sutapadey5274 I solve these problems for pure fun. Copying simply kills that fun, period. I looked for your solution now and found it in the twitter reply to Presh. I agree both our solutions are the same. I am way past high school and it took me a while to solve this. So, I don't view myself as creative.

[@vishalmishra3046]

This is much simpler using top/bottom triangles and sin(180-T)=sin T and using unknown angles B (180-4x) and D (180-8x) instead of A (8x=3x+5x) and C (4x=x+3x). Here it is, sin(B=180-4x)/sin(3x) = AC/BC = AC/CD = sin(D=180-8x)/sin(5x). So, sin(4x) sin(5x) = sin(3x) sin(8x). Rest identical. sin(5x) = sin(3x).2 cos(4x) = sin 7x - sin x. So sin x = sin 7x - sin 5x = 2 sin x cos 6x. So, cos 6x = 1/2 = cos 60 => x = 60/6 = 10 deg.

[@leiflawhite6890]

Are we totally ignoring the solution of x=0?

[@utkarshraj9651]

Taking x=0 leads to the figure to converge from a quadrilateral but according to question it is a quadrilateral so you can avoid x=0.

[@pierreabbat6157]

x=0 is indeterminate; the top and bottom corners of the trapezoid are the intersections of a line with itself. The only way to assign values to them is to take the limit; if you do that, you find that one is 3/4 of the way and the other is 5/8 of the way, not the same distance from the right corner.

[@someguy2267]

Yeah you are right, 90-x=90-3x x=0 can someone tell that

[@deadboy4735]

B=180-4x; D=180-8x; if(x==0){B=180; D=180} else if(x==5){B=160; D=140} else if(x==10){B=140; D=100} else if(x==15){B=120; D=60} else if(x==20){B=100; D=20} //Bonus question: How does length AC depend from angles B and D, if we know that BC=CD?

[@MarieAnne.]

Yes we are.

[@mahindaranasinghe9438]

i think we can copulate abc and abd triangles as bc=dc ,ac=ac ,3x=3x . So 2(3x+x+5x)=180 9x×2=180 180x=180 x=10

[@descendedsvurname6118]

It's the only video that i could understand in the whole presh's video

[@nobodyeew3467]

Why is it isosceles since the angles of the corners are not the same? 2:03 it will be x=3x, CD will be more long since it has bigger angle.

[@larisamedovaya9097]

4x+angle B=180; 8x+angle D=180;it means angle D=angle B/2; Let angle D=z then angle B=2z; 2z=180-4x; z=180-8x;then z=(180-4x)/2=90-2x; 180-8x=90-2x; 90=6x; x=15; angle B=180-4x=180-4*15=180-60=120 Degree ; angle D=180-8x=180-8*15=180-120=60; Our conclusion angle B =2angle D is correct. Solution to the problem is x=15 Degree .

[@wolffromnever4758]

B=180-4x; D=180-8x; B/2=90-2x; D is not equal to B/2. Your proof is based on a false statement.

[@HeckaS]

solved with trig in less then 10 min. it was an archaic brute force approach tho. got lucky by plugging in 10 degrees as value for x on second attempt. both triangles gave the same value for line ac and that was it.

[@jinhuiliao1137]

In the top triangle: AC/BC=sin(180-4x)/sin3x In the bottom triangle: AC/DC=sin(180-8x)/sin5x BC=DC sin(180-4x)/sin3x=sin(180-8x)/sin5x The rest is the same.

[@BriceLavorel]

That is a beautiful solution but if I may, it is missing some hypothesis to avoid undesired slutions. For example, you divide by sin(4x) without checking that sin(4x) is different from 0, and cos(6x)=1/2 has strictly 12 answers. It is obvious that 8x is strictly lower than 360° and greater than 0°, allowing to eliminate all those problems and having 1 solution. Great channel.

[@mrberry3483]

I've created a video showing how to do this problem using only basic geometry (no trigonometry required). kzbin.info/www/bejne/r6nTk5qkbLGLd7c Enjoy.

[@MarieAnne.]

At 3:40, after constructing an isosceles triangle, and figuring out that AB is parallel to CD, and therefore ∠CBD = ∠CDB = ∠ABD = y, and then using Law of Sines in 3 different triangle, you come to the conclusion that sin(8x)/sin(4x) = sin(5x)/sin(3x). However, there is a much more direct way to find this which only requires the use of the Law of Sine in 2 triangles. In △ABC, BC/AC = sin(3x)/sin(180-3x-x) = sin(3x)/sin(180-4x) = sin(3x)/sin(4x) In △ADC, CD/AC = sin(5x)/sin(180-3x-5x) = sin(5x)/sin(180-8x) = sin(5x)/sin(8x) Since BC = CD, then BC/AC = CD/AC ---> sin(3x)/sin(4x) = sin(5x)/sin(8x) which is equivalent to sin(8x)/sin(4x) = sin(5x)/sin(3x)

[@dmtri1974]

A nice problem. I do not think there is a reason to draw extra lines...Just use the sin rule for the triangles: ABC and ADC. Notice that the angles B and D are respectively: π-4x and π-8x....Then proceed as in the video. The difficult part (at least for me ) was to solve the trigonometric equation sin(5x)=2 cos (4x) sin(3x).... or equivalently the sin5x = sin7x +sin(-x). One last think, sin(4x) cannot be 0, as then x=π/4 and so 5x>π, which is not possible for a triangle.

[@TheFlax33]

Tough for me 1st time. i appreciate this channel. thanks

[@naveenreddy9141]

But in upper two triangles 3x+y+right angle=180° And x+y+right angle=180° Those two are not equal can u explain about this?

[@yurianlagrand5699]

I noticed this too, it seems like the upper right y is incorrect but also like that’s not used anywhere in the solution anymore so it can just be ignored. (This is what I think, I’m not an expert)

[@Guillermo90r]

Not sure if it is due to the drawing or what, but it looks like angle D is 90 degrees and due to AB being parallel to CD, angle A would also be 90 degrees. Therefore could you not divide 90 by 8 to get X? It gives 11.25 which is different to the answer given, thus my question.

[@jadaboud]

it is very interesting problem it through me of the graphic that you had it is misleading (AC is not perpendicular to BD otherwise x=3x ) Thank you for this brain teasers and keep going

[@shimmisuja5404]

Very easy way and interesting😃😃 only legend can solve this type of problems u r genius great 💓

[@georgexomeritakis2793]

There is a faster way to solve the problem, from point A draw a line segment AE parallel to BC. Then triangle ADE is isosceles with equal angles 4x, and DE = AD. Also EC = AB and finally AD+AB = BC = CD. From the triangles ADE and ABC on can derive that cos(4x)/cos(8x) + cos(x)/cos(3x) = 1, and therefore determine x = pi/18

[@mike1024.]

Hmm, using the law of sines when not solving a triangle. That's great! Careful on that step where you cancelled sin(x). You don't want that to be 0, but it's safe since x is strictly between 0 and 180 (and smaller).

[@greece8785]

Teacher: Did you read geometry Me:Yes Teacher: Ok solve this Me: So easy... Teacher: ...... Me after 5 mins:(😫😫😫😫😫😫)

[@manoharsinghshekhawat9272]

I got it!!!

[@shadrana1]

The best way to solve this problem is to use the Rule of Sines in triangle ABC and triangle ADC. Then use a series of trig identities to work out the only real solution for x. The best geometrical solution is to create an equilateral triangle within the ABCD and you end up with 6x=60 with x=10 degrees.When you end up with cos(6x)=1/2 from the trig method 6x lies within 0 and 180 deg.therefore x has to be 10.

[@shanmugasundaram9688]

Powerful application of trigonometric,law of sines, addition formula and product formula in a geometry problem.Very interesting.

[@Crabbi5]

Here was my scuffed solution: I set BC = 1, because the size doesn't matter Law of sines: sin(180º - 4x)/AC = sin(3x)/1 and sin(180º - 8x)/AC = sin(5x) => sin(8x)*sin(3x) = sin(4x)*sin(5x). Then I use that sin(8x) = 2*sin(4x)*cos(4x), and divide by sin(4x) like you did => 2cos(4x)*sin(3x) = sin(5x) Then I write everything as exponential functions, and simplify ( e**(-4ix) + e**(4ix) )*( 1/2*i*e**(3ix) - 1/2*i*e**(3ix) ) = 1/2*i*e**(-5ix) - 1/2*i*e**(5ix) 1/2*i*e**(-7ix) - 1/2*i*e**(7ix) + 1/2*i*e**(ix) - 1/2*i*e**(-ix) - 1/2*i*e**(-5ix) + 1/2*i*e**(5ix) = 0 I multiply by e**(7ix) to remove negative exponents, and divide by -1/2*i since it's a common factor in all terms -1 + e**(14ix) - e**(8ix) + e**(6ix) + e**(2ix) - e**(12ix) = 0 I define y = e**(2ix) and get a polynomial equation -1 + y**7 - y**4 + y**3 + y - y**6 = 0 y**7 - y**6 - y**4 + y**3 + y - 1 = 0 I see that y = 1 is a solution, but that only gives me x = 0, which technically is a solution, but not exactly the answer we want, since we'll no longer be dealing with a quadrilateral, but instead a line segment. However, since y=1 is a solution, it means I can divide the polynomial by (y -1). (y**7 - y**6 - y**4 + y**3 + y - 1)/(y - 1) = 0/(y - 1) y**6 - y**3 + 1 = 0 I then make another substitution z = y**3 = e**(6ix) z**2 - z + 1 = 0 z = 1/2 ± √(1 - 4)/2 z = 1/2 ± i*√3/2. Here I see that |z| = 1, so it can be written on the form e**(i*arg(z)) arg(z) = arctan((±√3/2)/(1/2)) = arctan(±√3) = ±60º z = e**(i*±60º) e**(6ix) = e**(i*±60º) 6x = ±60º x = ±10º. And since x is an angle in a triangle, x≥ 0, so x = 10º.

[@idk7016]

Crabbi5 that’s pretty good

[@kara6as]

There is the more simply way. Let E is the point on BC that the angle EAC is "x". It can be proved that the triangle AED has equal sides. Use the fact, that an external angle of triangle is equal to sum of opposite internal angles. Surely, it is easyer way and no trigonometric formulae.

[@Izumi_-de7jl]

The circle approach came to my mind, but i knew it ain't fair

[@MathMadeEasy]

Hi Presh, I love your channel and I try to replicate your video style. I know you probably won’t read this but do you have any advice on how to grow a small math channel like mine. Thanks Presh

[@SenthilKumar-qq5te]

I will hit a like to push your comment up. good luck.

[@MathMadeEasy]

@@SenthilKumar-qq5te Thanks

[@flutcubasahmet1303]

Me: This question is too hard! Presh: Hold my beer, fool. 😂

[@someguy2267]

Aha sonunda 1 türk gördüm kanalda

[@avitolourenco]

Opposite angles of a quadrilateral are 180⁰, so 12x=180?

[@Qermaq]

We don't know if it's convex tho.

[@shivanshsanoria4053]

That is only true if the quadrilateral is inscribed in a circle

[@amarkmr78]

*for cyclic quadrilaterals only.

[@RajinderKumar-ub4xn]

Why cant we do with help of congurency of triangles Step 1 BC=CD AC=AC COMMON angel BAC=angel ACD hence two triangles are congurent by SAS prorperty Hence angel DAC =angel ABC and angel BCA =angel ADC now in qudilateral all anels sum is 360 so angel A+angel B+ angel C+ angel D=360 8x+5x+4x+x=360 X=20 But i have doen every thing correct what is wrong here

[@nickvanamburg]

Wait, if BCD is isosceles, shouldn't AC go straight down the middle?

[@Whisky_Bravo]

looking at the figure at 2:46 how is it possible for 3x+y+90=180 to be true at the same time as x+y+90=180?

[@Aman-br1ph]

Why don't you use dark mode. I couldn't get it yet! 🤔 Don't anybody think the same...

[@idk7016]

agree

[@indigoziona]

I realised the shape was a trapezium, handed myself a small prize, then watched for the solution I was never going to get alone...

[@movieclips8809]

One question for math lover:Constructed a circle let's say that AB is the diameter of circle and O is the centre of circle now again constructed angleASB and angleAPB are two semi-circle angle and join PS let K is the point on diameter and from P join PK line while joining PK line another point make on line AS let called it F so given that angleAPK=angles PSA then show PK is perpendicular to AB and SFKB is cyclic quadrilateral by using circle theorem.

[@andrewwoan]

Much easier solution here. 5 x 3 = 15 15 x 3 = 45 45/5 = 9 9/3 = 3 x=10

[@nirmankhan2134]

🤣🤣🤣

[@anmaralt1921]

Possibly a simpler approach. Consider the triangle BDC. The angle BCD is (x + 3x). As BC = CD, if we change the angel BCA to 3x as well this would result into BCD as a equilateral triangle, then the angle BCD has to be 2 * 3x = 60 degree => x =10 degree

[@harithmenon2939]

Actually, you don’t need to use sin law. You can present y as a function of x . It can save a lot of time because you can calculate the sum of all angles in the quadrilateral (360 degrees) and equate the sum of all the angles in the quadrilateral to 360, before using algebra to find the value of X.

[@fewwiggle]

"It can save a lot of time" It saves so much time that there is no reason to even write it out, right? ;-)

[@PuzzleAdda]

How can we get 60 by adding only three numbers out of these: 2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50, 54, & 58? #PuzzleAdda

[@martinfan7209]

You can't, as all numbers are in the form 4ai + 2, the sum of any three is 4(a1+a2+a3) + 6 which is not a multiple of 4.

[@ahmedal-khouja450]

Wait but, cos(6x)=1/2 therefore there are 2 solutions for x, either pi/18 which is 10° and 5pi/18 which is 50°. Is there a reason why he didn't include the 2nd solution?

[@shivanshsanoria4053]

If x =50° then angle DAB=8x=400° which is not possible to have in a triangle

[@ahmedal-khouja450]

@@shivanshsanoria4053 my god why didn't I think of that 🤦🏻‍♂️thank you tho!!

[@Grizzly01]

@@ahmedal-khouja450 5:16

[@anshsingh4091]

I think this problem is totally wrong because BC is not equal to DC and Triangle BOC And DOC are proved to be congruent and they are not seen to be congruent.

[@arifahmad-7261]

5x+3x+3x+x=180⁰ (becoz sum of opposite angles of a quadrilateral =180⁰) => x=180⁰/12 =>x=15⁰

[@jonasriliskis996]

2:08 I dont understand how CBD (y) angle can be equal to BDC (y). They should not be equal - if they were and BC length is equal to CD length then BCA angle would be equal to ACD angle which they arent based on x and 3x in the picture

[@wolffromnever4758]

It's just that the quadrilateral is not drawn correctly. The CBD triangle is isosceles by condition, therefore, the segments AC and BD cannot intersect at right angles.

[@sinvidasinalma5979]

Aunque nadie me entienda Solo diré que la solución geométrica es menos tediosa

[@alejandroenciso9650]

Mira aquí : kzbin.info/www/bejne/r6nTk5qkbLGLd7c&feature=share

[@sinvidasinalma5979]

@@alejandroenciso9650 Interesante solución, no me imaginaba usar excentro para resolverlo. Yo lo hice prolongando "CB" y "DA" para formar un triangulo isosceles y aprovecharlo dándole una relación a sus lados seguido de dividir el ángulo "BAC" con una sebiana que corta al lado "BC" para formar otros dos isosceles que aprovechan la relación de lados antes mencionada, después se construye dentro del triangulo "BCD" el triangulo isosceles (congruente con el que construimos en el anterior paso, ambos de base "AC"), para provocar el típico caso de boomerang especial con relación de ángulos de una a dos y con 3 lados iguales. En lo personal me parece menos tediosa mi solución si es que sabes el caso del boomeran claro.

[@juanreyes1593]

Aquí una solución con solo geometría kzbin.info/www/bejne/q3iwqXZsppmnnbs

[@aungtoemyint5953]

(5x+3x)+(3x+x)=180 (sum of opposite angle ) 12x=180 x= 15 Please answer , my solution is correct or not?

[@anhdo2196]

Around 3:40 why are we using the formula for Sin (2A)? Doesn’t it, I don’t know how to word this, unbalanced the equation?

[@captaint8509]

BC = CD but literally unequally drawn

[@Wecoc1]

Yeah, that's typical

[@ujalavarshney480]

Yupp at first instance I also thought that.

[@rishurg2655]

Bro like the other side is longer makingnit like that

[@ThePowerfulOne07]

In geometry never assume the diagram is drawn to scale

[@ujalavarshney480]

@@ThePowerfulOne07 (Ik continuation) Until or unless it is stated in question. 😅😅😅😅 well said brother

[@JulianShagworthy]

Me - "Ah, there'll be a lovely, quick little trick to solve this!" Me at 4 minutes in - "Well that escalated quickly."

[@michachyra3513]

I think in a competition it should be noted sin x can be cancelles cause x is between 0 and 90 degrees, therefore sin x can't be 0.

[@saetainlatin]

Now I challenge you to solve it without any trigonometry, just euclidean geometry. That's what separates men from children

[@gianglanhtruong3069]

so true

[@felixfong3834]

The children are the ones who solve it with Euclidean geometry XD

[@fransschouten7997]

Triangle BCD is isosecles makingb angle CBD equal to CDB, also AB is parallel to CD because of the 3x angles making angle ADC : π-8x (π=180 dgs) so all angles can be expressed in terms of π and x Angle BDA is the smallest : π/2-6x en must be greater then 0 and thus x

[@GeomeTrazos]

See this version kzbin.info/www/bejne/fWOooHqXhdhribc

[@PotPoet]

Let us call the mid intersection point, E. There is no way that triangle BEC and triangle DEC are right triangles. Line BC and line CD are the same length and they are set at different angles so they cannot extend horizontally to the same extent. The horizontal vector of each line differs and thus no right angle would be formed between line BD and line AC. This presentation is in error right from the beginning.

[@JuanMataCFC]

at 5:16 u state that 0

[@CN_SFY_General]

Can you try x to be any angle between 1 and 14, for example x = 1, 2, 3, ..., 14? I think that these answers are all legal.

[@davidzegarra3386]

As a peruvian, I'm proud

[@yimmyherrera721]

Me too...!!greetings from Antofagasta ,Chile !!

[@alejandroenciso9650]

I am also Peruvian, I am as a monk now in India, many greetings!!!

[@alejandroenciso9650]

David mira aqui: kzbin.info/www/bejne/r6nTk5qkbLGLd7c&feature=share

[@GeomeTrazos]

David, esta versión geométrica (y peruana) te gustará kzbin.info/www/bejne/fWOooHqXhdhribc

[@chris2314]

How can be DBC = y ? Because y+90+x can't be equel to y+90+3x