定义法判单调：任取 x1<x2 比 f(x1) 与 f(x2) 大小；最值经单调性分析得出。
How many people do we need to have in a room to make it a favorable bet (probability of success greater than 1/2) tha…
一个教室里随便点 23 个人，两两生日撞上的概率刚过半。直觉猜 183，真答案是 23。
