#include <stdio.h> /* for input/output */
#include <math.h> /* for sin */ 

main() {
  float x;
  x = sin(30);
  printf("%f\n",x);
}

/* OUTPUT:
-0.988032
 */

/* NOTES:
Added
   #include <math.h>
...to correct problem. 
 */

/* NOTES:
Unlike the expectations of others, sin(30) will of course give 
sin(30 radians) not sin(30 degrees).
 */