sin(x+y)sin(x-y) adalah

Trigonometri.

sin α sin β = 1/2 [cos (α - β) - cos (α + β)]

sin (x + y) sin (x - y)
= 1/2 {cos [x + y - (x - y)] - cos [x + y + (x - y)]}
= 1/2 (cos 2y - cos 2x)