# Math Help - Trigonometric Identity

1. ## Trigonometric Identity

What trigonometric identity makes the following true?

$\sin(x)*\sin(y)=\frac{1}{2}[\cos(x-y)-\cos(x+y)]$

2. Originally Posted by Mike9182
What trigonometric identity makes the following true?

$\sin(x)*\sin(y)=\frac{1}{2}[\cos(x-y)-\cos(x+y)]$

$\frac{1}{2}[\cos(x-y)-\cos(x+y)]$

use the sum\difference identity for cosine ...

$\frac{1}{2}[\cos{x}\cos{y} + \sin{x}\sin{y} - (\cos{x}\cos{y} - \sin{x}\sin{y})]$