ActionScript for Flash MX: The Definitive Guide, 2nd Edition By Colin Moock
Chapter 18. ActionScript Language Reference
Math.acos( ) Method
Flash 5; can be used when exporting Flash 4 movies
compute the arc cosine of a number
Math.acos(x)
Arguments
x
A number between -1.0 and 1.0 (the cosine of an angle).
Returns
The angle, in radians, whose cosine is x. If x is not in the range -1.0 to 1.0, returns NaN.
Description
The arc cosine function (sometimes written as cos-1) is the inverse of the cosine function. It returns the angle whose cosine has the specified value, in radians. The return value is in the range of 0 to (i.e., 0 to 3.14159).