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28 August, 22:07

Simplify the expression by using a double-angle formula.

cos-39° - sin²39

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  1. 29 August, 00:07
    0
    cos 78⁰.

    Step-by-step explanation:

    Use cos 2A = cos^2 A - sin^2 A

    so cos^2 39 - sin^2 39 = cos (2*39) = cos 78.
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