X must be in radians. "To convert degrees to radians, multiply degrees by pi/180."
Derived Math Functions
The following is a list of nonintrinsic math functions that can be derived from the intrinsic math functions:
Function Derived equivalents
Inverse Cosine Arccos(X) = Atn(-X / Sqr(-X * X + 1)) + 2 * Atn(1)
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Atn Function
Returns a Double specifying the arctangent of a number.
Syntax
Atn(number)
The required number argument is a Double or any valid numeric expression.
Remarks
The Atn function takes the ratio of two sides of a right triangle (number) and returns the corresponding angle in radians. The ratio is the length of the side opposite the angle divided by the length of the side adjacent to the angle.
The range of the result is -pi/2 to pi/2 radians.
To convert degrees to radians, multiply degrees by pi/180. To convert radians to degrees, multiply radians by 180/pi.