Question

what is cos of negative pi/3?
what is sin of negative pi/3?

Answer 1

pi/3 is equivalent to 60 degrees, as 2pi is equal to 360 degrees. cos(60) in a triangle yields 1/2, and sin(60) yields (3^(1/2))/2. Thus, -pi/3, or -60 degrees would be a fourth quadrant point on the unit circle and these values would be negative as well, at cos(-pi/3)=-1/2 and sin(-pi/3)=-(3^(1/2))/2 

Answer 2

Cosine is even function, so:

$\cos(-\frac{\pi}{3})=\cos(\frac{\pi}{3})=\frac{1}{2}$

and sine is odd function, so:

$\sin(-\frac{\pi}{3})=-\sin(\frac{\pi}{3})=-\frac{\sqrt{3}}{2}$