#include<iostream>
#include<string>
#include<cmath.h>      //哪里不对？没有这个函数，在DEV C++4.9.9.2里？  
using namespace std;
int main()
{
    cout<<"Coefficients for quadratic equation: ";
    double a;
    double b;
    double c;
    cin>>a>>b>>c;
    if((a!=0)&&((b*b-4*a*c)>0)){
         double radical=sqrt(b*b-4*a*c);   //它说sqrt没有定义，奇怪了
         double root1=(-b+radical)/(2*a);
         double root2=(-b-radical)/(2*a);
         cout<<"The roots of "<<a<<"x**2+"<<b
         <<"x+"<<c<<" are "<<root1<<" and "<<
         root2<<endl;
    }
    else {
         cout<<"It has no two real roots."<<endl;
         }
    system("pause");
    return 0;
}

那为什么删除了   .h    后又能运行呢？    这是一个求平方根的 题目

谢谢