The hardest problem on the hardest test

延伸阅读 1

在球面四点形成的四面体问题里，先选穿心直线，再用公平硬币决定端点。这样仍是原来的均匀抽样，却把包住球心的事件变成可以数的符号选择。

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补充字幕原文 2

That first line passes through the sphere at two points, so flip a coin to decide which of those two points is going to be p1.

So one and only one of these eight equally likely outcomes gives us a tetrahedron that contains the center.