Angles that are measured relative to the positive direction of the x axis are positive if they are measured in the counterclockwise direction, and negative if measured clockwise.
Angles may be measured in degrees or radians (rad)
p rad = 180°
When the inverse trig functions sin-l, cos-l, and tan-1 are taken on a calculator, you must consider the reasonableness of the answer you get, because there is usually another possible answer that the calculator does not give. The range of operation for a calculator in taking each inverse trig function is indicated by the red curve in the following Figures.