ATAN

ATAN returns a floating-point value that is an approximation of the inverse (arc) tangent in radians of x or of a ratio x/y.

x and y

Expressions.

If x alone is specified, the result has the base and precision of x, and is in the range:

If x and y are specified, each must be real. An error exists if x and y are both zero. The result for all other values of x and y has the precision of the longer argument, a base determined by the rules for expressions, and a value given by: