Question

pleas help

Answer 1

do u know
\[2 \cos A \cos B ~=~\cos(A+B) + \cos(A - B)\]

Answer 2

yes

Answer 3

2 cos A cos B = cos (A + B) + cos (A - B)
cos A cos B = 1/2 (cos(A - B) + (cos A + B))

let A = pi/4
let B = pi/6

Then it will become