Question

If cos (A + B) = a cos A cos B + B sin A sinB,
then (a, b) is
(A) (-1,-1) (B) (-1,1)
(C) (1,-1)
(D) (1,1)​

Answer 1

Answer:

C

Step-by-step explanation:

I don't know how to prove it but it's a formula given in my textbook