##### Synopsis

    #include <math.h>
    double tan(double x);

##### Description

tan returns the radian circular tangent of x.

##### Fast math library behaviour

If |x| > 10^9, errno is set to EDOM and tan returns HUGE_VAL.

##### IEC 60559 math library behaviour

If x is NaN, tan returns x.
If |x| is ?, tan returns NaN with invalid signal.

##### Portability

tan conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).