trig
posted by rick .
Prove the identity
sin (3 pi/2 + x) + sin (3 pi/2 x) = 2 cos x

Use the identities
sin (a + b) = sin a cos b + sin b cos a
sin (a  b) = sin a cos b  sin b cos a
and the facts that sin 3 pi/2 = 1 and cos 3 pi/2 = 0