Question

Express cos5mcos(-3m) as a sum or difference.

Answer 1

Answer:

D

Step-by-step explanation:

Using the product to sum formula

• 2cosAcosB = cos(A+B) + cos(A - B)

note that cos(- 3m) = cos 3m, hence

cos5m cos(- 3m)

= cos 5m cos3m ← A = 5m and B = 3m

= $\frac{1}{2}$ ( 5m + 3m) + cos(5m - 3m) ]

= $\frac{1}{2}$ cos 8m + $\frac{1}{2}$ cos 2m