μ 的定义感觉这里其实还是没那么严谨的，Kulla Conty 原作里面注释说 μ 就是 cos(θ)：
Notice that I’m using the Greek letter µ for the cos θ term here.
This is just to avoid too much trigonometry in the slides and
it’s closer to how the formulas are actually implemented.
E_avg 我理解也应该是不需要做 sample 的，因为已经有了加权平均的过程……
