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