用牛顿迭代法，自己实现sqrt(和abs)函数功能

#include<stdio.h>
int main()
{
	double abs(double num);
	double sqrt(double numb);
	double x,y;
	printf("请输入要开方的数（必须为正数！）:");
	scanf("%lf",&x);
	while(x<=0)
	{
		printf("非法输入！请重新输入:");
		scanf("%lf",&x);	
	}
	y=sqrt(x);
	printf("\n结果为：sqrt(%5.4lf)=%17.16lf\n",x,y);
	return 0;
} 

double abs(double num)
{
	double ans;
	if(num<0) ans=-num;
	else ans=num;
	return(ans);
}

double sqrt(double numb)
{
	double x1,x2,x3,x4,x5,f,fx,c;
	double abs(double num);
	for(x1=0.1,x2=0,x3=0,x4=0,x5=0;;)
	{
		f=x1*x1-numb;
		fx=2.0*x1;
		x2=x1-f/fx;
		x5=x4;
		x4=x3;
		x3=x1;
		x1=x2;
		if(abs(x2-x5)<0.000000000001) break;
	}
	return(x1);
}