# 1. 题目

http://acm.timus.ru/problem.aspx?space=1&num=1753

## 1753. Bookshelf

Time limit: 0.5 second
Memory limit: 64 MB
A bookshelf has length L and borders of height h. A book of height H is standing vertically at its left side. Bibliophile gnomes want to drop the book onto the shelf. For that, they pull the lower edge of the book to the right along the surface of the shelf. The left side of the book remains leaning on the left border of the shelf. The gnomes pull the book until it lies horizontally on the shelf. However, the gravity may get in their way: if the center of the book gets too far beyond the left edge of the shelf, the book may overturn and fall to the floor. Let us assume that the book, shelf, and borders of the shelf have zero width. It is required to find how far to the left the center of the book can get in the process of the motion.

### Input

The only input line contains the integers h, H, and L (1 ≤ h < H < L ≤ 1000).

### Output

Output the value of the maximal displacement to the left of the center of the book with respect to the left side of the shelf accurate up to at least 10−5.

### Sample

input output
7 120 200
39.850361
Problem Author: Dmitry Ivankov
Problem Source: XI USU Open Personal Contest (March 13, 2010)

# 3. 代码

#include <stdio.h>
#include <math.h>

#define PI 3.1415926535897932384626433832795
#define ERROR 0.000001

double cal(int h, int H, double angle);
void solveE5d_Bookshelf();

int main() {
solveE5d_Bookshelf();
return 0;
}

void solveE5d_Bookshelf() {
int h, H, l;
scanf("%d %d %d", &h, &H, &l);
if (h >= H) {
printf("0\n");
return;
}

double left = PI / 2.0, right = 0, mid = (left + right) / 2.0, result, c;

while (left - right > ERROR) {
c = cal(h, H, mid);
if (c > 0) {
right = mid;
}
else {
left = mid;
}
mid = (left + right) / 2.0;
}

result = H / 2.0 * cos(mid) - h / tan(mid);
printf("%.10lf\n", result > 0 ? result : 0);
}

double cal(int h, int H, double angle) {
double c = cos(angle), s = sin(angle);
return (-1 * s * H / 2.0 + h / (s*s));
}