When the angle is in standard position, which quadrant does the terminal side fall?
Find an angle coterminal with x.
Find the reference angle for x.
Find the exact value of cos(x).
Any angle 2/3 pi with 2pi is coterminal 8/3 pi
or subtract 2 pi from it - 4/3 pi
reference angle is pi /3
cos is negative in Q II cos pi/3 is 1/2
so cos 2/3 pi = - 1/2