The _ACOS function returns the angle measured in radians based on an input COSine value ranging from -1 to 1.

## Syntax

radian_angle! = _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 COSine, which lets us turn a COSine 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 or _FLOAT precision variables instead of 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:

```
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)
```

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