To calculate the arctangent of an angle, follow these steps:

1. Determine the ratio of the opposite side to the adjacent side of the right triangle containing the angle you want to find. This is typically denoted as “opposite/adjacent” or “y/x”.
2. Enter the ratio into your calculator.
3. Press the “2nd” button on your calculator, followed by the “tan” button. This will give you the angle whose tangent is the ratio you entered in step 2.
4. The result will be in radians. If you want the answer in degrees, press the “2nd” button followed by the “degree” button.

The formula for the arctan function is:

arctan(x) = tan^(-1)(x)

where x is the input value to the arctan function. The result of the function is the angle (in radians) whose tangent is x.