# Math Help - 9 sin(2α) sin(3α)

1. ## 9 sin(2α) sin(3α)

This math problem has been giving me a headache. I understand the product-to-sum formula to be: sin u * sin v = (1/2)[cos(u-v)-cos(u+v)], but when I work out the problem I get: (9/2)(-cos(a) - cos(5a)). Is that wrong?

2. Originally Posted by sparker2000
This math problem has been giving me a headache. I understand the product-to-sum formula to be: sin u * sin v = (1/2)[cos(u-v)-cos(u+v)], but when I work out the problem I get: (9/2)(-cos(a) - cos(5a)). Is that wrong?
correct solution but

$\cos (-a) = \cos a$

$\frac{9}{2} \left( \cos (2a-3a) - \cos (2a+3a) \right)$
$\frac{9}{2} \left( \cos (-a) - \cos (5a) \right)$
$\frac{9}{2} \left( \cos (a) - \cos (5a) \right)$