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


{{PageSyntax}}
: {{Parameter|radian_angle!}} = [[_ACOS]]({{Parameter|cosine_value!}})

{{PageDescription}}
* 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]]ine, which lets us turn a [[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]] or [[_FLOAT]] precision variables instead of [[SINGLE]].


==Availability==
* '''Version 1.000 and up.'''


{{PageExamples}}
''Example:'' Converting a radian angle to its COSine and using that value to find the angle in degrees again using _ACOS:
{{CodeStart}} '' ''
{{Cl|DEFDBL}} A-Z

{{Cl|INPUT}} "Give me an Angle (in Degrees) => "; Angle
{{Cl|PRINT}}
C = {{Cl|COS}}({{Cl|_D2R}}(Angle)) '_D2R is the command to convert Degrees to Radians, which is what COS expects
{{Cl|PRINT}} "The COSINE of the Angle is: "; C
A = {{Cl|_ACOS}}(C)
{{Cl|PRINT}} "The ACOS of "; C; " is: "; A
{{Cl|PRINT}} "Notice, A is the Angle in Radians.  If we convert it to degrees, the value is "; {{Cl|_R2D}}(A) '' ''
{{CodeEnd}}
{{small|Example by SMcNeill}}
{{OutputStart}}
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
{{OutputEnd}}


{{PageSeeAlso}}
* [[_D2G]] {{text|(degree to gradient}}, [[_D2R]] {{text|(degree to radian)}}
* [[_G2D]] {{text|(gradient to degree)}}, [[_G2R]] {{text|(gradient to degree}}
* [[_R2D]] {{text|(radian to degree)}}, [[_R2G]] {{text|(radian to gradient}}
* [[COS]] {{text|(cosine)}}, [[SIN]] {{text|(sine)}}, [[TAN]] {{text|(tangent)}}
* [[_ASIN]] {{text|(arc sine)}}, [[ATN]] {{text|(arc tangent)}}
* [[_ACOSH]] {{text|(arc hyperbolic  cosine)}}, [[_ASINH]] {{text|(arc hyperbolic  sine)}}, [[_ATANH]] {{text|(arc hyperbolic  tangent)}}
* [[_ATAN2]] {{text|(Compute arc tangent with two parameters)}}
* [[_HYPOT]] {{text|(hypotenuse)}}
*[[Mathematical Operations]]
*[[Mathematical_Operations#Derived_Mathematical_Functions|Derived Mathematical Functions]]


{{PageNavigation}}