arctan
The arctan function is the inverse of the tangent function.
It returns the angle whose tangent is a given number.
Try this
Drag any
vertex of the triangle and see how the angle C is calculated using the arctan() function.
For every trigonometry function, there is an inverse function that works in reverse.
These inverse functions have the same name but with 'arc' in front.
(On some calculators the arctan button may be labelled atan, or sometimes
tan^{1}.)
So the inverse of tan is arctan etc. When we see "arctan x", we understand it as "the angle whose tangent is x"
tan 30 = 0.577 
Means: The tangent of 30 degrees is 0.577 
arctan 0.577 = 30 
Means: The angle whose tangent is 0.577 is 30 degrees. 
Use arctan when you know the tangent of an angle and want to know the actual angle.
See also Inverse functions  trigonometry
Example  using arctan to find an angle
In the above figure, click on 'reset'.
We know the side lengths but need to find the measure of angle C.
We know that
so we need to know the angle whose tangent is 0.577, or formally:
Using a calculator we find arctan 0.577 is 30°.
Large and negative angles
Recall that we can apply
trig functions to any angle, including large and negative angles. But when we
consider the inverse function we run into a problem, because there are an infinite number of angles that have the same tangent.
For example 45° and 360+45° would have the same tangent. For more on this see
Inverse trigonometric functions.
To solve this problem, the
range
of inverse trig functions are limited
in such a way that the inverse functions are onetoone, that is, there is only one result for each input value.
Range and domain of arctan
Recall that the domain of a function is the set of allowable inputs to it. The range is the set of possible outputs.
For y = arctan x :
Range 

Domain 
All real numbers 
By convention, the range of arctan is limited to 90° to +90° *.
So if you use a calculator to solve say arctan 0.55, out of the infinite number of possibilities it would return 28.81°,
the one in the range of the function.
* Actually, 90° and +90° are themselves not in the range. This is because the tan function has a value of
infinity at those values. But the values just below them are in the range, for example +89.9999999. But for simplicity of explanation,
we say the range is ±90° .
Things to try
 In the figure above, click 'reset' and 'hide details'.
 Adjust the triangle to a new size
 Using the arctan function calculate the value of angle C from the side lengths
 Click 'show details' to check the answer.
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Other trigonometry topics
Angles
Trigonometric functions
Solving trigonometry problems
Calculus
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