Acos
Function 
Acos(number)

Acos is included by default in each file as part of the Math library.
See also:
Use
The Acos() function returns the arccosine of a given unitless number. The result is given in radians, but it can be converted to degrees using "to deg".
Acos() also works with complex numbers.
Examples of trig arc functions
Acos(pi/4) = .667 rad
Acos(0.5) to deg = 60 deg
Atan(3ft/5ft) = 0.540 rad
Asin(1 + 2i) = (0.427 + 1.529 i) rad