Sum and Difference Formulas

Question

Express cos(60°)cos(-20°)-sin(60°)sin(-20°) as a function of a single angle.

Answer 1

cos(A+B)=cosAcosB-sinAsinB is a trigonometric identity, always true

substitute A=60 B=-20

cos(60-20)=cos60cos(-20)-sin60sin(-20)

=cos(40)