Question

Use the formula for cos(A-B) to derive a formula for cos(A+B)

Answer 1

cos (a - b) = cos a cos b + sin a sin b
cos (a + b) = cos (a - (-b)) = cos a cos (-b) + sin a sin (-b) = cos a cos b - sin a sin b
Formulas used:
cos (-x) = cos x
sin (-x) = - sin x

Answer 2

gr8 stuff @TURITW :)