Calculates the inverse hyperbolic cosine of x.
Remember to link the math library: valac -X -lm ...
That is the value whose hyperbolic cosine is x. If x is less than 1.0,
GLib.Math.acosh returns not-a-number float.NAN and
GLib.errno is EDOM.
| x |
A numeric value which is equal or greater than 1.0. |