Posted by **Kevin** on Sunday, November 13, 2011 at 4:51pm.

I have these sum and product property problems and I just want to make sure my work is right.

Write equations expressing

1. cos 37degrees cos99degrees as a sum

Answer: sin136+(-62)

2. sin4+sin7 as a product

Answer: 2sin5.5 sin-1.5

3. cos 8x - cos 2x as a product

Answer -2sin5x sin3x

4. cos5x sin9x as a sum

Answer sin14x - (-4)

Idk...I just don't feel as confident about these

- Trig -
**Steve**, Monday, November 14, 2011 at 10:35am
recall that

cosa * cosb = 1/2[cos(a+b) + cos(a-b)]

cos37 cos99 = 1/2(cos126 + cos(-62))

= 1/2 (cos126 + cos62)

since cos(-a) = cos(a)

So, your answers have the right number sin them, but you need to tidy them up to conform to the identities.

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