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Math Help - Finding cos(2A) given sin(A)

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    Question Finding cos(2A) given sin(A)

    If A is a positive acute angle and sin A = (square root) 5/3, what is cos 2A ?
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    Quote Originally Posted by Fenix Flaust View Post
    If A is a positive acute angle and sin A = (square root) 5/3, what is cos 2A ?
    \cos (2A) = 1 - 2 \sin^2 A.
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