后验概率解释

引用维基百科的[例子](https://zh.wikipedia.org/wiki/%E5%90%8E%E9%AA%8C%E6%A6%82%E7%8E%87)

# 理解
在穿裤子的条件下, 问是女生的概率: 等于穿裤子的概率, 分之既穿裤子又是女生的概率. 分子更进一步缩小范围.
```
P(是女生/看见穿裤子) = P(女生穿裤子)*P(女生)/P(穿裤子的概率)
= ... / [P(女)*P(裤/女)+P(男)*P(裤/男)]
```
# 记忆
集合乘法: 
`P(A/B)*P(B) = P(A∩B) = P(B/A)*P(A) ` 
两边同时除以P(B), 即为贝叶斯公式.

# 实际应用
[SSR撕逼](https://github.com/breakwa11/shadowsocks-rss/issues/868#issuecomment-317421860). 果然实际应用才是学习知识的唯一动力. <概率论>就应该这样学.

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