Question

How do you evaluate `cos(pi/3)cos((5pi)/12)-sin(pi/3)sin((5pi)12)`?

Answer 1

`-1/sqrt2.`

Explanation:

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

So, given Exp. `=cos(pi/3+5pi/12)=cos(9pi/12)=cos(3pi/4)=cos (pi-pi/4)=-cos(pi/4)=-1/sqrt2.`