standard.C.math.acos

acos is available since version 1.0.

Prototype:

number acos(number cosine)

Parameters

cosine
[in] Value whose arccosine is to be calculated, where -1 <= cosine <= 1.

Description:

Returns the arccosine of cosine in the range 0 to M_PI/2 radians.
If cosine is less than -1 or greater than 1, acos returns an indefinite by default.

Return value:

Returns the arccosine of cosine in the range 0 to M_PI/2 radians.

Example
import standard.C.math

class Main {
     function Main() {
          var x = 0;
          echo "Arcsine of " + x + " is " + asin(x) + "\n";
          echo "Arccosine of " + x + " is " + acos(x);
     }
} 
 

Results 
Arcsine of 0 is 0
Arccosine of 0 is 1.5707963267948966