A famous trig equation.

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  • เผยแพร่เมื่อ 16 ม.ค. 2025

ความคิดเห็น • 98

  • @goodplacetostop2973
    @goodplacetostop2973 4 ปีที่แล้ว +59

    13:05 10+1 = 11
    18:16

    • @muckchorris9745
      @muckchorris9745 4 ปีที่แล้ว +4

      Waiting for (michael penn)^300 xD

    • @vedants.vispute77
      @vedants.vispute77 4 ปีที่แล้ว +3

      Video 3 mins ago and this one 5 mins ago. Explain.

    • @goodplacetostop2973
      @goodplacetostop2973 4 ปีที่แล้ว

      @@muckchorris9745 As a matter of fact, blackpenredpen suggested me a great idea for another video. I’m open to more suggestions 😀

    • @goodplacetostop2973
      @goodplacetostop2973 4 ปีที่แล้ว +2

      @@vedants.vispute77 Video goes up at 8AM EST. So I just wait for that exact time and posts the comment. Really that simple.
      But I’ve noticed the bell notification goes up 2-3 minutes after the actual upload of the video.

    • @vedants.vispute77
      @vedants.vispute77 4 ปีที่แล้ว

      @@goodplacetostop2973 yeah ur right

  • @MizardXYT
    @MizardXYT 4 ปีที่แล้ว +23

    tan(2𝜋/3)+4sin(𝜋/3) = √3
    tan(4𝜋/7)+4sin(𝜋/7) = √7
    tan(4𝜋/11)+4sin(𝜋/11) = √11

  • @monikaherath7505
    @monikaherath7505 4 ปีที่แล้ว +24

    How do people come up with identities like this? Do they just stumble upon it while solving a problem or just by messing around? It's an extremely unintuitive identity with an astoundingly elegant proof using complex roots of unity. Honestly my mind is completely blown knowing that you could have precisely calculated this identity hundreds of years ago using just a pencil and paper. Question: are the trigonometric components in this identity transcendental while its sum is algebraic?

    • @kingkartabyo6206
      @kingkartabyo6206 4 ปีที่แล้ว +5

      Regarding your last comment, no. Both terms are in fact algebraic. Say s=sin 2pi/11, then using formula for sin(11x) in terms of sin x, we get: 0=sin 2pi=a polynomial in s.

    • @monikaherath7505
      @monikaherath7505 4 ปีที่แล้ว +1

      @@kingkartabyo6206 Nice, thank you.

    • @carlosgiovanardi8197
      @carlosgiovanardi8197 4 ปีที่แล้ว +4

      If you are interested in these kind of expressions, see arxiv.org/pdf/0709.3755.pdf and math.stackexchange.com/questions/45144
      Piece of cake!

  • @skysong8140
    @skysong8140 4 ปีที่แล้ว +26

    (A-B) can be solved by using the relation
    (A-B)^2 = (A+B)^2 - 4AB
    Would end up with two solutions but the negative one can be eliminated using the same logic as in the video.

    • @mathsandsciencechannel
      @mathsandsciencechannel 4 ปีที่แล้ว +1

      Thank you sir. you have encouraged me to start my maths and science channel. lets learn more and sub to be notified. thanks

  • @dhwyll
    @dhwyll 4 ปีที่แล้ว +7

    It took me a while to figure out the factorization at 2:00.
    e^(i 20/11 pi) = e^(i pi (22/11 - 2/11)) = e^(i pi 22/11 - i pi 2/11) = e^(i pi 22/11) * e^(-i pi 2/11) = e^(i 2 pi) * e^(-i 2 pi/11)

  • @laszloliptak611
    @laszloliptak611 4 ปีที่แล้ว

    At 9:57 you want to refer to the geometric series summation formula:
    1+x+x^2+...+x^n=(1-x^(n+1))/(1-x)
    Substituting x = -z^8 and n=9 we get
    1-z^8+z^16- ... -z^72 = (1 -(-z^72)(-z^8))/(1-(-z^8))=(1-z^80)/(1+z^8).
    Same formula is used at 14:10.

  • @karangupta1825
    @karangupta1825 4 ปีที่แล้ว +2

    It came on my TH-cam home screen as a suggestion and couldn't resist 'not to click on it'

  • @hsjkdsgd
    @hsjkdsgd 4 ปีที่แล้ว +7

    I didn't know I had picked a tough problem to solve on my own.

  • @n8cantor
    @n8cantor 4 ปีที่แล้ว +4

    Is there any deeper reason as to why the cross terms in AB add up to 2(A+B)? I don't see any pattern in the exponents and the cross terms in other groupings don't seem to add up to anything special. For instance, in the 2nd tool equation, the z terms also come in inverse pairs (i.e. z^7 and z^4 are in separate groups and z^7*z^4 = 1) but if you multiply them out, the cross terms are a mess.

  • @mihaipuiu6231
    @mihaipuiu6231 ปีที่แล้ว +1

    Very, very interesting equation. Good for you Penn. But very hard equation.

  • @Cypekeh
    @Cypekeh 4 ปีที่แล้ว +1

    your editing is subtly amazing

  • @markregev1651
    @markregev1651 4 ปีที่แล้ว +6

    That's the same idea as in IMO 1963? show that cos(pi/7)-cos(2pi/7)+cos(3pi/7)=1/2
    Oh and another suggestion for a geometry video: Fuss' theorem about bicentric quadrilaterals

    • @mathsandsciencechannel
      @mathsandsciencechannel 4 ปีที่แล้ว

      Thank you sir. you have encouraged me to start my maths and science channel. lets learn more and sub to be notified. thanks

    • @elkincampos3804
      @elkincampos3804 4 ปีที่แล้ว

      cos(π/7)=-cos(6*π/7) and cos(3*π/7)=-cos(4*π/7). Now, multiply the equation by 2. Take w=e^i(2*π/7). And use that 2*cos(x)=e^(i* x)+e^(-i*x).
      It follows the equality because w+w^2+w^3+w^4+w^5+w^6=-1.

  • @georgeebberson6
    @georgeebberson6 4 ปีที่แล้ว +2

    At 12:20, what happens to the 2 out front of the first "tool" expression? The i gets factored out, but rather than putting 2*z^10 inside the brackets (as well as 2*z) there's just z^10 and z respectively?

    • @athenaP24
      @athenaP24 4 ปีที่แล้ว +2

      You have 2*z^10 from the first tool, and - z^10 from the second tool, so you get z^10.

    • @georgeebberson6
      @georgeebberson6 4 ปีที่แล้ว +2

      @@athenaP24 ohhhhh I'm an idiot thanks 😂

    • @mathsandsciencechannel
      @mathsandsciencechannel 4 ปีที่แล้ว

      Thank you sir. you have encouraged me to start my maths and science channel. lets learn more and sub to be notified. thanks

  • @manucitomx
    @manucitomx 4 ปีที่แล้ว +9

    And I thought this was going to be a walk in the park. I’ve been had again.

  • @vbcool83
    @vbcool83 4 ปีที่แล้ว

    Didn't quite get the step at 10:02 - how does the 1-(z^8)^10 directly factor out 1+z^8?
    1-(z^8)^10 = (1-((z^8)^5)^2) = (1-(z^8)^5)(1+(z^8)^5) = (1-z^8)(1+z^8+z^16+z^24+z^32)(1+z^8)(1-z^8+z^16-z^24+z^32)
    That's how 1+z^8 gets factored out.

  • @ssvemuri
    @ssvemuri 4 ปีที่แล้ว +1

    I am mesmerized by this result. Summing up two trig expressions can yield sqrt(11). VOW. just VOW.
    Using the same technique derive a closed form for (x-cube + x-squared - 2x-1) where x = 2cos(2pi/7)
    Hint: the answer is not an irrational number.

  • @MathMan271
    @MathMan271 4 ปีที่แล้ว

    wow great work Michael

  • @ThePhysicsMathsWizard
    @ThePhysicsMathsWizard 4 ปีที่แล้ว +5

    wow, nice identity !!

  • @nawusayipsunam1643
    @nawusayipsunam1643 4 ปีที่แล้ว

    Yes. It is superb.

  • @ernest3109
    @ernest3109 4 ปีที่แล้ว +2

    15:25 whaaat? why is the rest of the product equal to 2(a+b)?

    • @skillerror951
      @skillerror951 4 ปีที่แล้ว

      Фейспалм кирюш, все же очевидно

    • @ernest3109
      @ernest3109 4 ปีที่แล้ว

      @@skillerror951 мне не очевидно объясни

    • @jounik
      @jounik 4 ปีที่แล้ว

      It's just the pairwise sums of numbers 1-10 mod 11 where you pick one summand from A={10,8,7,6,2} and the other one from B={1,3,4,5,9}. Once you've set aside the 5 pairs that sum to 0, you get two each of the other values, i.e. two sets of A+B.

    • @ernest3109
      @ernest3109 4 ปีที่แล้ว

      @@jounik Still unclear. I guess I have to write down all remaining 20 parts to see the pattern. Probably I'm stupid.

    • @jounik
      @jounik 4 ปีที่แล้ว

      @@ernest3109 Not stupid and it's not exactly obvious, but it _is_ easy to verify which is all Michael said. I suppose there are some cyclic properties that would help see it directly but it's easier to just write them out. I know I did.

  • @jaa2174
    @jaa2174 4 ปีที่แล้ว +2

    I get lost at 10:00. I believe you, but I would love to see where that result is coming from.

    • @MathrillSohamJoshi
      @MathrillSohamJoshi 4 ปีที่แล้ว +4

      I think he first split 1-z^80 as (1+z^40)(1-z^40)
      using the 'difference of squares' formula.
      Because(40=8(5)) we can factorise
      (1+z^40) such that one of its factors is the denominator, simplifying it to z^32-z^24+ z^16-z^8+1.
      Finally, multiplying the obtained expression by 1-z^40 gives us
      1-z^8+z^16-z^24+z^32-z^40+z^48-
      z^56+z^64-z^72
      Hope this helps :)

    • @jaa2174
      @jaa2174 4 ปีที่แล้ว +1

      @@MathrillSohamJoshi Thanks!

    • @MathrillSohamJoshi
      @MathrillSohamJoshi 4 ปีที่แล้ว

      @@jaa2174Welcome :)

  • @안태영-g8w
    @안태영-g8w 2 ปีที่แล้ว

    I tried for the answer by using trigonometric identities.
    First, define a constant "θ = π/11".
    Let A = tan(3π/11)+4sin(2π/11) = tan(3θ)+4sin(2θ).
    Multiply both sides by cos(3θ) ⇒ Acos(3θ) = sin(3θ)+4sin(2θ)cos(3θ) = sin(3θ)+2sin(5θ)-2sin(θ).
    Then square both sides ⇒ (Acos(3θ))^{2} = [sin(3θ)+2sin(5θ)-2sinθ]^{2}
    and expand right hand side ⇒ RHS = sin^{2}(3θ)+4sin^{2}(5θ)+4sin^{2}(θ) + 4sin(3θ)sin(5θ)-8sin(5θ)sin(θ)-4sin(θ)sin(3θ)
    Use two identities: "sin^{2}(α) = (1-cos(2α))/2" and "sin(α)sin(β) = (cos(α-β)-cos(α+β))/2".
    Then, RHS = (1-cos(6θ))/2+2(1-cos(10θ))+2(1-cos(2θ)) + 2(cos(2θ)-cos(8θ))+4(cos(6θ)-cos(4θ))+2(cos(4θ)-sin(2θ))
    = 9/2 + (7/2)cos(6θ) -2(cos(2θ)+cos(4θ)+cos(8θ)+cos(10θ))
    Besides, bring another identity: "Σ{0≤k≤n-1}cos(2πk/n)=0".
    Substitute "n=11" and "θ = π/11" ⇒ 1+cos(2θ)+cos(4θ)+cos(6θ)+…+cos(20θ)=0
    And since "cos(α) = cos(2π-α) = cos(22θ-α)", it can be written as "1+2(cos(2θ)+cos(4θ)+cos(6θ)+cos(8θ)+cos(10θ))=0".
    Therefore, (Acos(3θ))^{2} = RHS = 11/2 + (11/2)cos(6θ) = 11cos^{2}(3θ), and A^{2}=11
    Since A>0, the answer is A = tan(3π/11)+4sin(2π/11) = sqrt(11).

  • @Zxv975
    @Zxv975 4 ปีที่แล้ว

    I'm confused, what happened to the 1+z^8 denominator around 10:30?

    • @zhaochengliu9970
      @zhaochengliu9970 4 ปีที่แล้ว +1

      I think that's the geometric series summation, but in an inverse manner. The ratio of the series is z.

    • @Zxv975
      @Zxv975 4 ปีที่แล้ว

      @@zhaochengliu9970 ohhhhhh I see it now! It's an alternating geometric sum, got it. Thanks so much for that. I was thinking about it completely wrong because Michael mentioned about generalised difference of squares and I just couldn't see how that related. Also, did you mean to say the ratio is z^8 (or rather, I guess -z^8).

    • @zhaochengliu9970
      @zhaochengliu9970 4 ปีที่แล้ว

      @@Zxv975 Sure haha! I was struggling on that point too XD. It's kinda confusing. And yep, you are right, the ratio is actually -z^8.

  • @Артем-х9у9к
    @Артем-х9у9к 4 ปีที่แล้ว +1

    Also tan(π/11)+4*sin(3π/11)=√11

  • @AlephThree
    @AlephThree 4 ปีที่แล้ว +1

    Brilliant!

  • @rbdgr8370
    @rbdgr8370 4 ปีที่แล้ว +2

    What was the motivation behind using 11th root of unity for solving sin(2pi/11) and 22nd root of unity for solving tan(3pi/11)?Was it the step involved at 7:56 or there is a certain algorithm for such problems??

    • @synaestheziac
      @synaestheziac 4 ปีที่แล้ว +1

      Seriously. This derivation had a whole bunch of steps I never would have thought of.

    • @rbdgr8370
      @rbdgr8370 4 ปีที่แล้ว

      @@synaestheziac after 7:56 it was comparatively easier as I have solved many ques on those concepts but before that I never would've guessed taking 22nd root of unity for solving tan(3π/11)

    • @synaestheziac
      @synaestheziac 4 ปีที่แล้ว

      @@rbdgr8370 I got befuddled as soon as he constructed z to the 88th power, and then rewrote it as z to the 8th to the 10th - can you help me see how one would decide to make those moves?

  • @sundeep0207
    @sundeep0207 4 ปีที่แล้ว +1

    Great approach.
    Is there a method without using complex numbers?

    • @get2113
      @get2113 4 ปีที่แล้ว +1

      Very likely, but that route would entail searching through many tables of trig identities, some of might even have been proved via Euler. See your point but shortcuts are often needed to make progress.

    • @sundeep0207
      @sundeep0207 4 ปีที่แล้ว +2

      @@get2113 Yes, an approach without using the complex numbers might be too complex 😁

    • @dulcedeleche000
      @dulcedeleche000 3 ปีที่แล้ว

      Yes its possible you can find the solution of this question in IIT JEE Amit M Agrrwalas book. And when I did this question in 2008 publication KC Sinha trigonometry and there was no solution in this book. And yes the solution was purely trigonometric no complex numbers concept used.

  • @claudioproy9489
    @claudioproy9489 4 ปีที่แล้ว +4

    Can you generalize this for any prime in denominator?

    • @BatmanPooping
      @BatmanPooping 4 ปีที่แล้ว +1

      Where was it used that 11 is prime?

    • @synaestheziac
      @synaestheziac 4 ปีที่แล้ว

      ​@@BatmanPooping I think it has to do with the way in which certain exponents are inverses of each other mod 11 and the way in which the cross terms in the product of A and B reproduce the original exponents. I'm just learning about primitive roots mod p in Michael's number theory videos, so I'm not entirely sure what I'm talking about, but it seems like there are number-theoretic results hovering in the background herer.

    • @BatmanPooping
      @BatmanPooping 4 ปีที่แล้ว +1

      @@synaestheziac No. It has nothing to do with being prime. For any natural n, e^(2 pi i (n-1)/n)= e^(-2 pi i/n).

  • @pattemsarvesh5002
    @pattemsarvesh5002 4 ปีที่แล้ว

    Great video 🙌

  • @amberdeshbhratar9342
    @amberdeshbhratar9342 4 ปีที่แล้ว +2

    How to prove
    1/cos(π/7) +
    2cos(π/7)/cos(2π/7) =4

    • @sundeep0207
      @sundeep0207 4 ปีที่แล้ว

      Was able to transform this problem to the below one. Proving one is equivalent to proving the other.
      Show that Cos(pi/7)+Cos(3pi/7)+cos(5pi/7)=0.5
      Got stuck here. Any thoughts on this?

    • @sundeep0207
      @sundeep0207 4 ปีที่แล้ว +2

      Was able to arrive at the proof. Please find it below.
      Part 1 of the proof
      Show that the given problem is equivalent to proving
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) = 0.5
      Consider X = 1/Cos(π/7) + 2Cos(π/7)/Cos(2π/7) - 4.
      Our original problem is proving that X=0.
      Multiplying both sides by Cos(π/7)*Cos(2π/7), we get
      Cos(π/7)*Cos(2π/7)*X = Cos(2π/7) + 2*Cos(π/7)*Cos(π/7) - 4*Cos(π/7)*Cos(2π/7)
      For the purpose of brevity, I will replace the LHS of the equation with Y.
      We can see that X=0 if and only if Y=0 . Hence, the given problem is same as showing Y=0.
      Y = Cos(2π/7) + 2*Cos(π/7)*Cos(π/7) - 4*Cos(π/7)*Cos(2π/7)
      Y = Cos(2π/7) + 2*Cos(π/7)*Cos(π/7) - 2*(2*Cos(π/7)*Cos(2π/7))
      Y = Cos(2π/7) + 2*Cos(π/7)*Cos(π/7) - 2*(Cos(3π/7)+Cos(π/7))
      Y = Cos(2π/7) + Cos(2π/7) + 1 - 2*(Cos(3π/7)+Cos(π/7))
      Y = 2*Cos(2π/7) + 1 - 2*(Cos(3π/7)+Cos(π/7))
      Y = -2*Cos(5π/7) + 1 - 2*(Cos(3π/7)+Cos(π/7))
      Therefore,
      Y = 1 - 2*(Cos(π/7) + Cos(3π/7) + Cos(5π/7))
      From this we can observe that
      Showing X=0 is same as showing
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) = 0.5
      Part 2 of the proof
      Prove that
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) = 0.5
      Consider the equation Z^7 + 1 = 0
      The roots of this equation are β, β^3, β^5, β^7, β^9, β^11 and β^13 , where β = e^(iπ)/7
      We can see that the sum of the roots of this equation is 0.
      i.e., β + β^3 + β^5 + β^7 + β^9 + β^11 + β^13 = 0
      Considering the real parts on both sides, we get
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) + Cos(7π/7) + Cos(9π/7) + Cos(11π/7) + Cos(13π/7) = 0
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) + (-1) + Cos(9π/7) + Cos(11π/7) + Cos(13π/7) = 0
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) + Cos(9π/7) + Cos(11π/7) + Cos(13π/7) = 1
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) + Cos(5π/7) + Cos(3π/7) + Cos(π/7) = 1
      2*(Cos(π/7) + Cos(3π/7) + Cos(5π/7)) = 1
      Therefore,
      Cos(π/7) + Cos(3π/7) + Cos(5π/7) = 0.5
      Please let me know if something is not clear.

    • @amberdeshbhratar9342
      @amberdeshbhratar9342 4 ปีที่แล้ว

      thank you 😁

  • @guill3978
    @guill3978 4 ปีที่แล้ว

    Is 1/ln(2) + 1/ln(3) a transcendental number?

    • @nerdiconium1365
      @nerdiconium1365 4 ปีที่แล้ว

      From knowing e is transcendental, you can figure out each addend is at least irrational, but this feels like it might be in the same vein as “We don’t know if pi+e is rational, irrational, or transcendental” types of questions. Also, the reciprocals make it harder because it effectively switches the argument and the base of the logarithm.

  • @shailendragupta7517
    @shailendragupta7517 4 ปีที่แล้ว

    This can be done without using complex number

  • @yashagrahari
    @yashagrahari 4 ปีที่แล้ว

    It would seem more magical if you don't see down to the floor.

  • @l0new0lf000
    @l0new0lf000 4 ปีที่แล้ว

    Nice hoodie

  • @megacarlosloki
    @megacarlosloki 4 ปีที่แล้ว

    Try OBMU second fase problem 5 :)

    • @sofly666
      @sofly666 4 ปีที่แล้ว

      esse daí é brabo, né não

    • @lawlietl1370
      @lawlietl1370 4 ปีที่แล้ว

      De qual ano?

    • @lawlietl1370
      @lawlietl1370 4 ปีที่แล้ว

      @@sofly666 nada melhor do que mais um BR por aqui

  • @eusterich3035
    @eusterich3035 4 ปีที่แล้ว

    All iit jee aspirant just rote learn it 😂😂😂

  • @eusterich3035
    @eusterich3035 4 ปีที่แล้ว

    Yeah kinda like memorise shit

  • @TrueBagPipeRock
    @TrueBagPipeRock 4 ปีที่แล้ว

    k

  • @mathsandsciencechannel
    @mathsandsciencechannel 4 ปีที่แล้ว +1

    Thank you sir. you have encouraged me to start my maths and science channel. lets learn more and sub to be notified. thanks

  • @vedants.vispute77
    @vedants.vispute77 4 ปีที่แล้ว +5

    All people here listen,
    Prove that the probability of getting a like on this video is zero.
    Disproove this statement.

    • @Thaplayer1209
      @Thaplayer1209 4 ปีที่แล้ว +3

      There are 2 dislikes now what have you done

    • @goodplacetostop2973
      @goodplacetostop2973 4 ปีที่แล้ว

      Asking people on the internet to not do something is the best way to provoke that thing 😛

    • @vedants.vispute77
      @vedants.vispute77 4 ปีที่แล้ว

      @@drsonaligupta75 I saved myself.