Question

What is the exact value of `cos155˚cos35˚ + sin155˚sin35˚`?

Answer 1

We have

`cos(A - B) = cosAcosB + sinAsinB`

Matching, we get that `alpha = 155` and `beta = 35`.

This means that the expression is in fact equivalent to

`cos(120˚) = -1/2`

Hopefully this helps!