The OpenD Programming Language

acos

Calculates the arc cosine of x, returning a value ranging from 0 to π.

Special Values
x	acos(x)	invalid?
>1.0	NaN	yes
<-1.0	NaN	yes
NaN	NaN	yes

real acos(real x)
double acos(double x)
@safe pure nothrow @nogc
double
acos
(
double x
)
float acos(float x)

Examples

import std.math.operations : isClose;
import std.math.traits : isNaN;
import std.math.constants : PI;

assert(acos(0.0).isClose(1.570796327));
assert(acos(0.5).isClose(PI / 3));
assert(acos(PI).isNaN);

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Source

See Implementation

std math trigonometry functions