# trig

posted by rick

Prove the identity
sin (3 pi/2 + x) + sin (3 pi/2 -x) = -2 cos x

1. drwls

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

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