hihoCoder #1142 : 三分·三分求极值

#include<cmath> 
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#define eps 1e-7 
using namespace std;
double a,b,c,x,y;
double l,r,mid1,mid2;
double f(double xx){
    double yy=a*xx*xx+b*xx+c;
    double dis=sqrt((xx-x)*(xx-x)+(yy-y)*(yy-y));
    return dis;
}
int main(){
    scanf("%lf%lf%lf%lf%lf",&a,&b,&c,&x,&y);
    l=-200,r=200;
    while(r-l>eps){
        mid1=(l+r)/2;
        mid2=(mid1+r)/2;
        if(f(mid1)<=f(mid2))    r=mid2;
        else  l=mid1;
    }
    printf("%.3lf",f(l));
}