* The arc-tangent is the inverse of the TAN function.
* This function is also called the inverse tangent function.
* The "number" is the angle in radians.
* If "number" is not numeric, then #VALUE! is returned.
* The returned angle is given in radians between -PI/2 and PI/2.
* If your argument is in degrees, multiply it by PI/180 to convert it to radians.
* If you want to convert the result from radians to degrees, multiply it by 180/PI.
* You can use the TAN function to return the tangent.
* You can use the ACOS function to return the arc-cosine.
* You can use the ASIN function to return the arc-sine.
* You can use the DEGREES function to convert from radians to degrees.
* For a working example refer to the Trigonometric Functions page.
* For the Microsoft documentation refer to support.office.com