#include<iostream>
#include<stdio.h>

int main(void){

    double a;
    int i = -1;
        double NUM [100000];

     while(EOF != scanf("%lf", &a)){
      NUM[++i] = sqrt(a);
     }

     i--;
     while(i>=0){
         printf("%.4f\n",NUM[i]);
         i--;
     }
     return 0;

}

Have you less a header file <math.h>?