A Problem about Polyline

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题目：

There is a polyline going through points (0, 0) – (x, x) – (2x, 0) – (3x, x) – (4x, 0) – ... - (2kx, 0) – (2kx + x, x) – ....

We know that the polyline passes through the point (a, b). Find minimum positive valuex such that it is true or determine that there is no suchx.

题意：一条以x为比例因子渐增的折线，给一个点判断该点能否出现在该折线上，以及如果能，求最小的x

分析：数学题，主要是判断之后，找到点左右的两个折线与x轴相交的点（其实是两种情况，在折线的上升沿和下降沿），然后分别作为最小情况求出x的值，比较后留下最小的。

题解：

#include <iostream>#include <cstdio>#include <cstdlib>#include <algorithm>#include <queue>#include <stack>#include <vector>#include <map>#include <string>#include <cstring>#include <functional>#include <cmath>#include <cctype>#include <cfloat>#include <climits>#include <complex>#include <deque>#include <list>#include <set>#include <utility>#define rt return 0#define fr freopen("in.txt","r",stdin)#define fw freopen("out.txt","w",stdin)using namespace std;double wr(int a, int b){int m = a / (2 * b);return (double)a / (double)(2 * m);}int main(){//fr;int a, b;cin >> a >> b;if (a<b){cout << -1 << endl;rt;}int t = a - b;int p = a + b;double ans = min(wr(p, b), wr(t, b));printf("%.10lf\n", ans);rt;}