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{{unreferenced|date=August 2012}}
{{dablink|This article is about a mathematical term. For an entity in computer science, see [[Coroutine]].}}

In [[mathematics]], a [[function (mathematics)|function]] ''f'' is '''cofunction''' of a function ''g'' if ''f''(''A'') = ''g''(''B'') whenever ''A'' and ''B'' are [[complementary angles]]. This definition typically applies to [[trigonometric functions]].

For example, sine and cosine are cofunctions of each other (hence the "co" in "cosine"):

{| class="wikitable"
|-
| <math>\sin\left(\frac{\pi}{2} - A\right) = \cos(A)</math>
| <math>\cos\left(\frac{\pi}{2} - A\right) = \sin(A)</math>
|-
|}

The same is true of secant and cosecant and of tangent and cotangent:

{| class="wikitable"
|-
| <math>\sec\left(\frac{\pi}{2} - A\right) = \csc(A)</math>
| <math>\csc\left(\frac{\pi}{2} - A\right) = \sec(A)</math>
|-
| <math>\tan\left(\frac{\pi}{2} - A\right) = \cot(A)</math>
| <math>\cot\left(\frac{\pi}{2} - A\right) = \tan(A)</math>
|-
|}

Sometimes writing a function in terms of its cofunction helps solve trigonometric equations. A simple example is the equation sin ''A'' = cos ''B''.

== See also ==
* [[Trigonometric function]]

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[[Category:Trigonometry]]

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en>Sadads: no longer orphaned