uva 11346 - Probability(概率)

option=com_onlinejudge&Itemid=8&page=show_problem&problem=2321">题目链接：uva 11346 - Probability

题目大意：给定x。y的范围。以及s。问说在该范围内选取一点。和x，y轴形成图形的面积大于s的概率。

解题思路：首先达到方程xy ≥ s，即y = s / x。

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S2的面积用积分计算，y = s / x的原函数为lnx
所以S2=s∗(ln(a)−ln(x))

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>

using namespace std;

int main  () {
    int cas;
    double a, b, s;
    scanf("%d", &cas);

    while (cas--) {
        scanf("%lf%lf%lf", &a, &b, &s);
        double r = min(s / b, a);
        double ans = r * b + log(a) * s;

        if (fabs(s) > 1e-9)
            ans = ans - log(r) * s;

        double p = 1 - ans / (a * b);
        printf("%.6lf%c\n",  fabs(p * 100), '%');
    }
    return 0;
}