Question

cos x cos y = 1/2(sin(x + y)+ sin(x-y)) is this true or false I went with false I was looking at the product sum

Answer 1

Which do you think it is?

Answer 2

\[\begin{cases}\cos (x+y)=\cos x\cos y-\sin x\sin y\\
\cos(x-y)=\cos x\cos y+\sin x\sin y\end{cases}\]
Adding the equations gives
\[\cos(x+y)+\cos(x-y)=2\cos x\cos y\]