Jika cosx =p dan cos y= q, maka hitunglah sin 1/2(x+y) sin1/2(x-y)

cos x = p
cos y = q

sin (x + y)/2 sin (x - y)/2
= -1/2 (cos x - cos y)
= -1/2 (p - q)
= 1/2 (q - p)