九度oj 1551 二分 + 精度

#include<iostream>
#include<stdlib.h>
#include<stdio.h>
#include<math.h>
#include<string.h>
#include<string>
#include<queue>
#include<algorithm>
#define PI acos(-1.0)
#define eps 1e-9
using namespace std;
double R;
double area(double h)
{
    double arc=asin(h*0.5/R);
    return R*R*arc- 0.5 *h*sqrt(R*R - 0.25*h*h);
}
double are(){return R*R*PI;}

double judge(double h)
{
    return area(h)/(are()-area(h));
}
double bin_search(double r)
{
    double left ,right, mid;
    left=0,right=2*R;
    double S=are();
    while(right-left>eps)
    {
        mid=(left+right)/2;
        if(judge(mid) - r> eps )
            right=mid-eps;
        else left = mid+eps;
    }
    return (left+right)/2.0;

}
int main()
{
    double r;
    while(cin>>R>>r)
    {
        printf("%.2f\n",bin_search(r));
    }
    return 0;
}