sin (A + B) = sin A + cos B sin (A - B) = sin A - cos B
From there it's just a matter of simplification.
sin A cos B = 1/2 [ sin (A + B) + sin (A - B) ] sin A cos B = 1/2 [ sin A cos B + cos A sin B + sin (A - B) ] sin A cos B = 1/2 [ sin A cos B + cos A sin B + sin A cos B - cos A sin B ] sin A cos B = 1/2 [ 2 sin A cos B ] sin A cos B = sin A cos B