I would convert -2pi / 3 to degrees first by multiplying by 180degrees / pi. It equals 120 degrees.
120 degrees is in quadrant II, which is 90 degrees plus 30 degrees. It is 60 degrees from the 180 degrees.
So the reference triangle you use is 60 degrees.
Cos60 = 0.5
sin60 = sqrt(3) /2
But in quadrant II the only trig ratio that's positive is sin; all others are negative.
So cos120 = -0.5
sin60 = sqrt(3) /2
Keeping in mind that points on unit circle are (cos, sin), the point is (-0.5, sqrt(3) /2).
