acos Arc cosine &reftitle.description; floatacos floatnum Returns the arc cosine of num in radians. acos is the inverse function of cos, which means that a==cos(acos(a)) for every value of a that is within acos' range. &reftitle.parameters; num The argument to process &reftitle.returnvalues; The arc cosine of num in radians. &reftitle.seealso; cos acosh asin atan