With inverse cosine, we select the angle on the top half of the unit circle. Thus cos-1(–½) = 120° or cos-1 (–½) = 2π/3. In other words, the range of cos-1 is restricted to [0, 180°] or [0, π]. Note: arccos refers to "arc cosine", or the radian measure of the arc on a circle corresponding to a given value of cosine. 
