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Homework Help: Double angle problem

  1. Oct 13, 2012 #1
    1-2cos2 25/ 1 -2sin2 65

    The attempt at a solution

    1-2cos^50= sin^2 50
    so sin^2 50 / cos^2 130

    I'm stuck and most likely this isn't correct.
    Last edited: Oct 13, 2012
  2. jcsd
  3. Oct 13, 2012 #2


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    Science Advisor

    What you have written makes no sense at all. Surely you don't believe that 2sin(x)= sin(2x)? What trig identities do you know?

    Are we to assume that the "25" and "65" are in degrees? Why not just use a calculator to evaluate them?
  4. Oct 13, 2012 #3
    Oh I can't use a calculator, that's what it says in the instructions. the identities I know are

    cos^2x + sin^2 = 1
    1+tan^2x = sec^2 x
    cot^2x + 1 = cosec^2x

    The addition formulae and the double angle formulae

    yes they are in degrees.
  5. Oct 13, 2012 #4
    I got it I think. I'm sorry for being stupid, I forgot about the double angle identities for sin^2 and cos^2

    I substitued them in and I got 1. Which I think is correct.
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