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Math Help - Compound Angles

  1. #1
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    Compound Angles

    If cosA = 3/5, 0deg <A<90deg write sinA as a fraction.
    Hence write cos2A as a fraction.

    I've got the first part sinA as a fraction, 4/5 . The second part, cos2A as a fraction is getting me. Any help?
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  2. #2
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    Quote Originally Posted by zyx.gar View Post
    If cosA = 3/5, 0deg <A<90deg write sinA as a fraction.
    Hence write cos2A as a fraction.

    I've got the first part sinA as a fraction, 4/5 . The second part, cos2A as a fraction is getting me. Any help?
    Hi zyx.gar,

    Try using the double angle identity:

    \cos 2A = \cos^2A-\sin^2A
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