Tangent (tan) function - Trigonometry

(See also Tangent to a circle).
In a right triangle, the tangent of an angle is the length of the opposite side divided by the length of the adjacent side.
Try this Drag any vertex of the triangle and see how the tangent of A and C are calculated.

The tangent function, along with sine and cosine, is one of the three most common trigonometric functions. In any right triangle, the tangent of an angle is the length of the opposite side (O) divided by the length of the adjacent side (A). In a formula, it is written simply as 'tan'.

As an example, let's say we want to find the tangent of angle C in the figure above (click 'reset' first). From the formula above we know that the tangent of an angle is the opposite side divided by the adjacent side. The opposite side is AB and has a length of 15. The adjacent side is BC with a length of 26. So we can write This division on the calculator comes out to 0.577. So we can say "The tangent of C is 0.5776 " or

Example - using tangent to find a side length

If we look at the general definition - we see that there are three variables: the measure of the angle x, and the lengths of the two sides (Opposite and Adjacent). So if we have any two of them, we can find the third.

In the figure above, click 'reset'. Imagine we didn't know the length of the side BC. We know that the tangent of A (60°) is the opposite side (26) divided by the adjacent side AB - the one we are trying to find. From our calculator we find that tan 60° is 1.733, so we can write Transposing: which comes out to 26, which matches the figure above.

The inverse tangent function - arctan

For every trigonometry function such as tan, there is an inverse function that works in reverse. These inverse functions have the same name but with 'arc' in front. So the inverse of tan is arctan etc.

When we see "arctan A", we interpret it as "the angle whose tangent is A"

tan 60 = 1.733 Means: The tangent of 60 degrees is 1.733
arctan 1.733 = 60 Means: The angle whose tangent is 1.733 is 60 degrees.

We use it when we know what the tangent of an angle is, and want to know the actual angle.

See also arctangent definition and Inverse functions - trigonometry

Large and negative angles

In a right triangle, the two variable angles are always less than 90° (See Interior angles of a triangle). But we can in fact find the tangent of any angle, no matter how large, and also the tangent of negative angles. For more on this see Functions of large and negative angles.

When used this way we can also graph the tangent function. See Graphing the tangent function.

The derivative of tan(x)

In calculus, the derivative of tan(x) is sec2(x). This means that at any value of x, the rate of change or slope of tan(x) is sec2(x).

For more on this see Derivatives of trigonometric functions together with the derivatives of other trig functions. See also the Calculus Table of Contents.

While you are here..

... I have a small favor to ask. Over the years we have used advertising to support the site so it can remain free for everyone. However, advertising revenue is falling and I have always hated the ads. So, would you go to Patreon and become a patron of the site? When we reach the goal I will remove all advertising from the site.

It only takes a minute and any amount would be greatly appreciated. Thank you for considering it!   – John Page

Become a patron of the site at

Other trigonometry topics


Trigonometric functions

Solving trigonometry problems