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Homework Help: Trigonometric functions (identity&equations)

  1. Apr 29, 2015 #1
    1) Problem: given that x is an obtuse angle for which cos^2x/(1 + 5sin^2x) = 8/35, find the value of cosx/(1 - 5 sin x) without evaluating x.

    2) Relevent equations:
    sin(-x) = - sin x
    cos(-x) = cos x
    sin(180° - x) = sin x
    cos(180° - x) = - cos x
    sin^2x + cos^2x = 1

    3) Attempt:
    cos^2x/(1 + 5sin^2x) = 8/35
    35cos^2x = 8 + 40sin^2x

    Actually I am clueless on how to tackle this problem, as in what should I be even doing to get to the answer.

    Please help, thank you!
  2. jcsd
  3. Apr 29, 2015 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Try combining the last of your 'relevant equations' with the last equation in your attempt.
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