how to solve sin^-1(cos(2pi/3))?
sin^-1(cos(2pi/3)
cos 2pi/3 = -1/2 ....so...
sin-1(-1/2) = -pi/6
Remember that the domain for inverse sine is $$-\pi/2\le \pi/2$$
So only 1st and 4th quadrant answers are relevant. :)