>>3
#include <stdio.h>

double sqr(double c);

int main(){
double c;
c = sqr(2);
printf("√2は%fです。\n", c);
return 0;
}

double sqr(double c){
double xn;
int i,n;

n = 10;
xn = 0;

do {
xn += 1;
} while (xn * xn < c);

for (i = 0;i < n;i++) {
xn = (xn + c / xn)/2;
}

return xn;
}

これでいいよ