ACOS function
From Microsoft Office ShapeSheet
ACOS function
Returns the arccosine of number, for example, the angle whose cosine is number.
Syntax
ACOS(number)
Remarks
The input value must be in the range -1 <= number <= 1, or a #NUM! error is returned. The resulting angle is in the range 0 <= angle <= PI radians (0 <= angle <= 180 degrees).
Example
ACOS(0)
Equals 90 deg.