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The difference cos 7x − cos 3x equals:
A−2 sin 5x sin 2x
B2 sin 5x sin 2x
C2 cos 5x cos 2x
D−2 cos 5x sin 2x
Answer & Solution
Correct answer: A. −2 sin 5x sin 2x
cos A − cos B = −2 sin((A+B)/2) sin((A−B)/2), so cos 7x − cos 3x = −2 sin 5x sin 2x.
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