double sin(double);

return the sinus

#include <iostream> /* including standard library */
#include <cmath> /* including math library */
 
using namespace std;
int main (void)
{
  double x;
  x = 42.0;
 
  //cout.setf(ios::fixed, ios::floatfield);
  cout.precision(8);
 
  cout << "sine of " << x << "° is " << sin(x*3.1415/80) << endl;;
  return 0;
}

  sine of 42° is 0.99692115