12-08-2019
Mathematics

contestada

Simplify the expression by using a double-angle formula.
cos-39°– sin²39

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13-08-2019

Answer:

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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