Question

Choose the right expression in terms of a single trigonometric function. cos space x space cos space 2 x space plus space sin space x space sin space 2 x

Answer 1

Are you saying
cosx cos(2x) + sinx sin(2x) ??? , then why not type it that way?

You should recognize the formula
cos(A - B) = cosAcosB + sinAsinB

cosx cos(2x) + sinx sin(2x)
= cos(x - 2x)
= cos(-x)
= cosx