#include <stdio.h>
#include <math.h>
#define START 100 // number to start with
int main(void){
	double number = START;
	double root;
	while(1){
		root = sqrt(number);
		printf("Square root of %lf is %lf.\n",number,root);
		number = root;

	}
    return 0;
}

root@killbox:~/Desktop/C# gcc -o lol 1.c && ./lol
/tmp/ccGdvAMz.o: In function `main':
1.c:(.text+0x42): undefined reference to `sqrt'
collect2: ld returned 1 exit status