use the unit circle to find the exact value for all the trig functions of è=2ð/3.
I do not understand your notation, è=2ð/3.
Do you mean theta = 2 pi/3?
The sine of 2 pi/3 is the same as the sine of pi/3, which is (sqrt3)/2
The cosine of 2 pi/3 has the same absolute value as cos pi/3 (1/2), but has opposite signposted by drwls