#include <math.h>
double cosh(double x);
cosh calculates the hyperbolic cosine of x.
If |x| >~ 709.782, errno is set to EDOM and cosh returns HUGE_VAL.
If x is +∞, -∞, or NaN, cosh returns |x|.
If |x| >~ 709.782, cosh returns +∞ or
-∞ depending upon the sign of x.
cosh conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).