`acosh`

, `acoshf`

---inverse hyperbolic cosine#include <math.h> double acosh(doublex); float acoshf(floatx);

**Description**

`acosh`

calculates the inverse hyperbolic cosine of `x`.
`acosh`

is defined as

`x` must be a number greater than or equal to 1.

`acoshf`

is identical, other than taking and returning floats.

**Returns**

`acosh`

and `acoshf`

return the calculated value. If `x`
less than 1, the return value is NaN and `errno`

is set to `EDOM`

.

You can change the error-handling behavior with the non-ANSI
`matherr`

function.

**Portability**

Neither `acosh`

nor `acoshf`

are ANSI C. They are not recommended
for portable programs.

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