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.