ACS returns the arc-cosine of a numeric expression whose value is between -1 and +1 inclusive. The arc-cosine of a number is the angle whose cosine is that number. ACS returns a value between 0 and PI radians or 180 degrees, depending on the current trigonometric mode. The default trigonometric mode is radians.
COMPLEX Arguments
ACS accepts either a COMPLEX or REAL argument and returns a value of the same type. For COMPLEX arguments the angle is returned in radians, regardless of the current trigonometric mode. ACS returns the principal value, defined (in terms of complex arithmetic) as
which returns a real part in the range 0 to PI. The domain for COMPLEX arguments includes all points in the complex plane (but for REAL arguments, the domain is still -1 to +1 inclusive). Notice that intermediate values generated during the calculation of the function can cause over or underflow errors for very large or small values of Z.