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?

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?