#include <math.h>
double cos(double x);
cos returns the radian circular cosine of x.
If |x| > 10^9, errno is set to EDOM and cos returns HUGE_VAL.
If x is NaN, cos returns x.
If |x| is ?, cos returns NaN with invalid signal.
cos conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).