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### 1.3 `acos`, `acosf`—arc cosine

Synopsis

    #include <math.h>
    double acos(double x);
    float acosf(float x);

Description

`acos` computes the inverse cosine (arc cosine) of the input value. Arguments to `acos` must be in the range -1 to 1.

`acosf` is identical to `acos`, except that it performs its calculations on `floats`.

Returns
`acos` and `acosf` return values in radians, in the range of 0 to pi.

If x is not between -1 and 1, the returned value is NaN (not a number) the global variable `errno` is set to `EDOM`, and a `DOMAIN error` message is sent as standard error output.

You can modify error handling for these functions using `matherr`.