//the functions take radians; M_PI is POSIX, not ISO C
double sin90 = sin(M_PI / 2);
//sin90 is 1
double cos180 = cos(M_PI);
//cos180 is -1
double tan45 = tan(M_PI / 4);
//tan45 is 0.99999999999999989
//degrees to radians
double degrees = 30;
double sin30 = sin(degrees * M_PI / 180);
//sin30 is 0.5
printf("sin90 is %g\n", sin90);
printf("cos180 is %g\n", cos180);
printf("tan45 is %.17g\n", tan45);
printf("sin30 is %g\n", sin30);