Question

Rewrite with only sin x and cos x.

cos 3x

A. cos x - 4 cos x sin^2 x

B. -sin^3 x + 2 sin x cos x

C. -sin^2 x + 2 sin x cos x

D. 2 sin^2 x cos x - 2 sin x cos x

Answer 1

Besides knowing trig identities and being able to use angle addition formulae, consider that only one of those functions has the proper behavior near x = 0.

Answer 2

Try cos(2x + x).

Use cos addition formula, and then cos double-angle formula.

Answer 3

http://www.wolframalpha.com/input/?i=cos+3x