3) atan2(sqrt(3)) which gives the result: 60deg, 240 deg

In all 3 cases, draw a circle with radius of 1. Remember that the convention is that angle grows in counterclockwise direction. 0deg and 360deg point due right, 90deg up, 180deg due left, 270 deg down. The sinus is the vertical distance (plus is above the horizontal axis, minus is below the horizontal axis.) The cosine is the distance from the vertical axis (i.e. it is the horizontal distance - do not get confused by this).