cos ( x - y ) sin ( x + y ) =....

COS(X-Y)SIN(X+Y)
(SINXSINY + COSXCOSY)(SINXSINY-COSXCOSY)
(X+Y)(X-Y) = X^2 - Y^2
(SINXSINY)^2- (COSXCOSY)2
COS(X-Y)SIN(X+Y) = SIN^2XSIN^2Y - COS^2XCOS^2Y

sifat trigonometri
2sinx.cosy = sin (x+y) + sin (x-y)

dari soal didapat a = x+y dan b = x-y, maka
sin a. cos b = 1/2 [sin (a+b) + sin (a-b)]
= 1/2 [sin (x+y+x-y) + sin (x+y-x+y)]
= 1/2 [sin 2x + sin 2y]
= 1/2[2sinxcosx + 2sinycosy]
= sinxcosx + sinycosy


maaf kalau salah. semoga membantu