1. ## extra credit

For extra credit I have to show that:

-5/39 sin8x sin5x - 8/39 cos8x cos5x = -cos13x/26 - cos3x/6.

Any suggestions on where to start? Thanks for the help.

2. Originally Posted by Carol
For extra credit I have to show that:

-5/39 sin8x sin5x - 8/39 cos8x cos5x = -cos13x/26 - cos3x/6.

Any suggestions on where to start? Thanks for the help.
From the compound angle formulae:

$\sin x \sin y = \frac{\cos(x - y) - \cos (x + y)}{2}$

$\cos x \cos y = \frac{\cos(x - y) + \cos (x + y)}{2}$