double tan(double);

Returns the tangent of a radian angle x.

compile with gcc tan.c -o tan -lm

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <math.h> /* including math library */
 
int main( void )
{
    double x;
    x = 42;
 
    printf ("tan of %lf° is %lf\n", x, tan (x*3.1415/180));
    return 0;
}

 output:
 user@host:~$ ./tan 
 tan of 42.000000° is 0.900365