Question

Evaluate

Answer 1

\[\int\limits_{}^{}\cos(2y)\cos(7y)dy\]

Answer 2

use the identity :
cosAcosB = 1/2 (cos(A+B) + cos(A-B))

Answer 3

so,
cos(2y)cos(7y) = 1/2 (cos(2y+7y) + cos(2y-7y)) = 1/2 (cos9y + cos5y)
note : cos(-5y) = cos(5y)

Answer 4

Thank you!

Answer 5

then the integration be simple for you
you're welcome :)