The [_ACOS](_ACOS) function returns the angle measured in radians based on an input [COS](COS)ine value ranging from -1 to 1.

## Syntax

> radian_angle! = [_ACOS](_ACOS)(cosine_value!)

## Description

* The *cosine_value!* must be measured >= -1 and <= 1, or an error will be generated.  (PRINT _ACOS(1.2) would give the result of -1.#IND, which is basically QB64's way of telling us that the number doesn't exist, much like 1/0 would.) 
* ARCCOSINE is the inverse function of [COS](COS)ine, which lets us turn a [COS](COS)ine value back into an angle.
* Note: Due to rounding with floating point math, the _ACOS may not always give a perfect match for the COS angle which generated this.  You can reduce the number of rounding errors by increasing the precision of your calculations by using [DOUBLE](DOUBLE) or [_FLOAT](_FLOAT) precision variables instead of [SINGLE](SINGLE).

## Availability

* Version 1.000 and up.

## Example(s)

Converting a radian angle to its COSine and using that value to find the angle in degrees again using _ACOS:

```vb

DEFDBL A-Z

INPUT "Give me an Angle (in Degrees) => "; Angle
PRINT
C = COS(_D2R(Angle)) '_D2R is the command to convert Degrees to Radians, which is what COS expects
PRINT "The COSINE of the Angle is: "; C
A = _ACOS(C)
PRINT "The ACOS of "; C; " is: "; A
PRINT "Notice, A is the Angle in Radians.  If we convert it to degrees, the value is "; _R2D(A) 

```

```text

Give me an Angle (in Degrees) => ? 60

The COSINE of the Angle is:  .5000000000000001
The ACOS of  .5000000000000001  is:  1.047197551196598
Notice, A is the Angle in Radians.  If we convert it to degrees, we discover the value is  60

```

## See Also

* [_D2G](_D2G) (degree to gradient, [_D2R](_D2R) (degree to radian)
* [_G2D](_G2D) (gradient to degree), [_G2R](_G2R) (gradient to degree
* [_R2D](_R2D) (radian to degree), [_R2G](_R2G) (radian to gradient
* [COS](COS) (cosine), [SIN](SIN) (sine), [TAN](TAN) (tangent)
* [_ASIN](_ASIN) (arc sine), [ATN](ATN) (arc tangent)
* [_ACOSH](_ACOSH) (arc hyperbolic  cosine), [_ASINH](_ASINH) (arc hyperbolic  sine), [_ATANH](_ATANH) (arc hyperbolic  tangent)
* [_ATAN2](_ATAN2) (Compute arc tangent with two parameters)
* [_HYPOT](_HYPOT) (hypotenuse)
*[Mathematical Operations](Mathematical-Operations)