Question

Why is cos(4pi/3) = - cos(pi/3)?

Answer 1

Please see below.

Explanation:

`cos(4pi/3)=cos(pi+pi/3)`

Now using `cos(A+B)=cosAcosB-sinAsinB`, the above is equal to

`cospicos(pi/3)-sinpisin(pi/3)`

= `(-1)xxcos(pi/3)-0xxsin(pi/3)`

= `-cos(pi/3)`