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# Power Pivot Principles: The A to Z of DAX Functions – ATAN

21 September 2021

In our long-established Power Pivot Principles articles, we are starting a new series on the A to Z of Data Analysis eXpression (DAX) functions. This week it’s ATAN without colour …

The ATAN function

This function returns the principal value of the arctangent (inverse tangent) of a number (which is not the same as the reciprocal of the tangent). This needs some explanation. Consider a right-angled triangle:

Using the age-old mathematics mnemonic SOHCAHTOA, we get:

• sin x = opposite / longest side (hypotenuse) = O/H
• cos x = adjacent / hypotenuse = A/H
• tan x = opposite / adjacent = O/A
• arctan y = tan-1 y is the angle whose tangent is y.

This may be charted as follows:

The arctangent is the angle whose tangent is number.  The returned angle is given in radians in the range (-pi/2) to (pi/2).

The syntax is very simple:

=ATAN (number)

It has only one argument:

• number: the tangent of the angle you want.