arc_cosine
This function returns the principal value of the arc cosine of x, expressed in radians.
double arc_cosine(double x)
Parameters:
x
The value to calculate between -1 and 1.
Return value:
Principal arc cosine of x, in the interval [0,pi] radians. If x is outside the valid range, the function will return 0.
Remarks:
In trigonometrics, arc cosine is the inverse operation of cosine.
Example:

    void main()
    {
    alert("arc_cosine test", "Arc Cosine of -1 is "+arc_cosine(-1)+".");
    alert("arc_cosine test", "Arc Cosine of 0 is "+arc_cosine(0)+".");
    alert("arc_cosine test", "Arc Cosine of 1 is "+arc_cosine(1)+".");
    }