Question

expressing this product as a sum
cos(5η) cos(3η)

Answer 1

would it be (1/2)[cos(2η) + cos(8η)]

Answer 2

these type of problems, you can always google for

Answer 3

\[\cos(A-B)=\cos A \cos B+\sin A \sin B ...(1)\]
\[\cos \left( A+B \right)=\cos A \cos B-\sin A \sin B ...(2)\]
adding (1) and (2)

Answer 4

\[2\cos A \cos B=\cos \left( A-B \right)+\cos \left( A+B \right)\]

Answer 5

yea so it should be (1/2)[cos(2η) + cos(8η)]

Answer 6

correct.