1. Determine an equivalent expression for sin 3x cos x + cos 3x sin x . A. 4sin x B. 2sin x cos x C. 4sin x cos x D. 2sin 2x cos 2x 2. sin(a+b) = sinacosb+cosasinb, sin2x = 2sinxcosx 3. This question appeared on a practice final I was attempting at. It's on a no calculator section so I'm having difficulty solving it without one. I got up to sin 3x cos x + cos 3x sin x = sin(3x + x) = sin(4x) The answer is 2sin 2x cos 2x, I know that sin2x = 2sinxcosx but I don't know how to expand it when sin2x becomes sin4x. It'd be great if someone could explain it. Thanks!