double sqrt(double);

returns the square root

#include <iostream> /* including standard library */
#include <cmath> /* including math library */
 
using namespace std;
int main ( void )
{
 
  //cout.setf(ios::fixed, ios::floatfield);
  cout.precision(8);
 
  cout << "square root from 4.2 is " << sqrt (4.2) << endl;
  return 0;
}

square root from 4.2 is 2.0493902