A Gorgeous Solution via Trig Substitution (WMC 70)

  • Published on Dec 13, 2018
  • Let's solve our system of equations using familiar trigonometric identities.
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Comments • 45

  • ThatOneGuy
    ThatOneGuy 2 months ago

    i solved this without too many trig subtitution by turning the first equation into circle equation (turning sin^2 + cos^2 into y^2 + x^2 = 1), then solve for x/y in the 2nd eq and sub the value back to 1st equation

  • Math Zone
    Math Zone 3 months ago

    I love your video usclip.net/channel/UCLFe_L2xHeL0JQlUi9MJ15g?view_as=subscriber thanks!

  • JidtapadTK
    JidtapadTK 3 months ago

    I scream extremely loud in my mind when I see you sub
    x = sin α
    Love you 🥰

  • Chetra Kea
    Chetra Kea 6 months ago

    Teacher please write up cause down in close by writing load

  • el tapa
    el tapa 6 months ago

    This was delightful

  • GreenMeansGO
    GreenMeansGO 7 months ago

    I hope people can read this and respond. I just solved this problem without even using the first equation(the one with two radicals). If we take
    25(1-y^2)=41-40sqrt(1-x^2) and let x=cos(θ) and y=sin(θ) them the equation turns into
    25(1-y^2)=41-40y implying that y=4/5 and x=3/5. Done.

    • GreenMeansGO
      GreenMeansGO 7 months ago

      Oh, wow. You are right. I got lucky. My solution assumes that (x,y) is a point on the unit circle which turned out to be correct in the end but was not necessarily true to begin with. Thank you for your feedback.

    • Ankush Agarwal
      Ankush Agarwal 7 months ago

      This is not something you can do immediately.
      Sinθ and cosθ are interrelated and substituting both of them in the equation for x and y is not right. In this question it just turned out to be correct. Thats not the case always as connection between x and y may not be the same as that of sinθ and cosθ.

  • Parth Chopra
    Parth Chopra 7 months ago

    This was amazing!
    Like next level clever...thanks mate!! :D

  • Shubham Saraf
    Shubham Saraf 8 months ago

    What do you mean by exclusive?

  • Lumina Mathavan
    Lumina Mathavan 8 months ago

    Good mental exercise. Thank you sir

  • Jeffrey Cloete
    Jeffrey Cloete 8 months ago

    That was beautiful. .thanks!

  • human :3
    human :3 8 months ago

    I can't help but notice That your "Hand"writing has become better over time; did you buy a drawing table?

  • Yerram Varun
    Yerram Varun 8 months ago

    That was awesome! Love your channel

  • Rajendra Misir
    Rajendra Misir 8 months ago

    Terrific job Prakhar Agarwal! I find it amazing that equation 1 can be rewritten as the equation of the unit circle using trigonometric identities such as the half angle of tangent function(tan(x/2).

  • gajra rangare
    gajra rangare 8 months ago

    Nice Problem.

  • Paul Murray
    Paul Murray 8 months ago

    Seeing sqrt(1-x^2), I was thinking that the way to go would be to redo the whole thing in polar coordinates.

  • dunkelheit
    dunkelheit 8 months ago

    Why has he used two different angles? Is it wrong to use only one? And if it is wrong, why is it wrong? Thanks for your time.

    • dunkelheit
      dunkelheit 7 months ago

      @π Super J π Thank you so much!

    • π Super J π
      π Super J π 8 months ago

      If you used the same angle then you would be assuming x and y are the same value because sine is bijective between 0 and 90.

  • Anandan Poornash
    Anandan Poornash 8 months ago +1

    very nice solution

  • el tapa
    el tapa 8 months ago

    Ahhhh very nice. I didn't knew that tangent identity

  • יהודה שמחה ולדמן

    Did you get my solution in email?

  • Shanmuga Sundaram
    Shanmuga Sundaram 8 months ago

    Very nice illustration.Thank you.

  • adandap
    adandap 8 months ago +12

    The trig solution is lovely. I wouldn't have thought of it myself, but worked it through after noticing it in Prakhar's answer. That said, the algebraic approach is nicer than it looks at first glance because of the way that (1 - sqrt(1-x^2))/x rationalises and inverts. And the y term simplifies nicely to Sqrt( (1+y)/(1-y)).

  • Slight Lokii
    Slight Lokii 8 months ago +3

    What is your reasoning that alpha and beta are complimentary?

    • Sea cucumber
      Sea cucumber 8 months ago

      @Slight Lokii Happy to help :)

    • Slight Lokii
      Slight Lokii 8 months ago +1

      Sea cucumber oh I understand now! Thanks :)

    • Sea cucumber
      Sea cucumber 8 months ago +5

      I'm going to use a for alpha and b for beta:
      tan a/2 = tan (45 - b/2)
      The following is a well known tangent identity:
      If tan x = tan y then x=y + k×360° where k is some whole number. Therefore:
      a/2 = 45 - b/2 + k×360°
      Rearranging gives you:
      a+b=90 + k×360°
      However, we set both a and b to be between 0° and 90° so we cannot add or subtract any whole number multiple of 360° except for 0. So:
      a+b=90° + 0×360°
      a+b=90° which is the definition of complementary angles.

  • Mi Les
    Mi Les 8 months ago +3

    It’s just too much algebra just to get nice answer lol

  • Antonio Banda
    Antonio Banda 8 months ago +5

    Face reveal please

  • Cristhian Martínez
    Cristhian Martínez 8 months ago +1


  • Ben Burdick
    Ben Burdick 8 months ago +7

    I feel really dumb for not seeing the trig sub when I attempted this problem.

  • Santhosh Kumar
    Santhosh Kumar 8 months ago +7

    Thank you very much for the solution. I am big fan of your channel.

    • Santhosh Kumar
      Santhosh Kumar 8 months ago

      Thank you for the heart . Don't stop uploading difficult problems with simple solutions. I am eagerly waiting for the next weekly challenge number 71 .

  • Pete Berg
    Pete Berg 8 months ago

    In what cases again i can use trig substitution?
    What if the question doesnt tell that 0

    • Jerry Tan
      Jerry Tan 8 months ago

      Just want to add a complement on Prathmesh's answer. In this specific question, notice that abs(x) and abs(y) has to be between 0 and 1 as sqrt(1-x^2) and sqrt(1-y^2) are both undefined when abs(x) or abs(y) is greater than 1 (you can't have a negative inside the square root for Real Numbers). Therefore, in this specific question, even if the information (0

    • Pete Berg
      Pete Berg 8 months ago +1

      @Prathmesh Joshi thx. That might be a clue

    • Prathmesh Joshi
      Prathmesh Joshi 8 months ago +1

      note that -1

  • Pete Berg
    Pete Berg 8 months ago

    Well, this open my mind. I hope i can apply this strategy as my tool

  • Ducksfan101
    Ducksfan101 8 months ago +54

    Okay that’s epic.

    • bridogg154
      bridogg154 8 months ago

      Solve this one usclip.net/video/m0PqvrZbcTo/video.html