The arccos function is the inverse of the cosine function.

It returns the angle whose cosine is a given number.

It returns the angle whose cosine is a given number.

Try this
Drag any
vertex of the triangle and see how the angle C is calculated using the arccos() 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 arccos button may be labelled acos, or sometimes
cos^{-1}.)
So the inverse of cos is arccos etc. When we see "arccos x", we understand it as "the angle whose cosine is x"

cos30 = 0.866 | Means: The cosine of 30 degrees is 0.866 |

arccos 0.866 = 30 | Means: The angle whose cosine is 0.866 is 30 degrees. |

See also Inverse functions - trigonometry

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 cosine is 0.866, or formally:
Using a calculator we find arccos 0.866 is 30°.

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 cosine. For example 45° and 360+45° would have the same cosine. 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 one-to-one, that is, there is only one result for each input value.

Recall that the domain of a function is the set of allowable inputs to it. The range is the set of possible outputs.

Range | |

Domain |

By convention, the range of arccos is limited to 0 to +180°. So if you use a calculator to solve say arccos 0.55, out of the infinite number of possibilities it would return 56.63°, the one in the range of the function.

- In the figure above, click 'reset' and 'hide details'.
- Adjust the triangle to a new size
- Using the arccos function calculate the value of angle C from the side lengths
- Click 'show details' to check the answer.

- Angle definition, properties of angles
- Standard position on an angle
- Initial side of an angle
- Terminal side of an angle
- Quadrantal angles
- Coterminal angles
- Reference angle

- Introduction to the six trig functions
- Functions of large and negative angles
- Inverse trig functions
- SOH CAH TOA memory aid
- Sine function (sin) in right triangles
- Inverse sine function (arcsin)
- Graphing the sine function
- Sine waves
- Cosine function (cos) in right triangles
- Inverse cosine function (arccos)
- Graphing the cosine function
- Tangent function (tan) in right triangles
- Inverse tangent function (arctan)
- Graphing the tangent function
- Cotangent function cot (in right triangles)
- Secant function sec (in right triangles)
- Cosecant function csc (in right triangles)

- The general approach
- Finding slant distance along a slope or ramp
- Finding the angle of a slope or ramp

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