The ATN function receives the ratio of 2 sides of a right triangle as an argument (number) and returns the corresponding angle. The 2 sides mentioned here include the right angle. The ratio of the 2 sides is the value of the opposite side length to the obtained angle divided by the adjacent side (base, or side adjacent to the obtained angle) length. The return value will range from -π/2 to π/2 (expressed in radians). To convert degrees to radians, multiply the degree by π/180. To convert radians to degrees, multiply the radian by 180/π.

The ATN function is an inverse trigonometric function of the Tan function. The Tan Function receives an angle as an argument, and returns the ratio of the 2 sides that include the right angle of a right triangle. Note the difference between the ATN function and the cotangent (1/tangent) of the reciprocal tangent.