方向向量 + 法向量：把垂直、夹角、距离全变成解方程。
Suppose that we have a coin which comes up heads with probability p, and tails with probability q. Now suppose th…
一枚偏币，正面的概率是 p。连扔两次，第一次正面和第二次反面——这两件事到底算不算独立？
