正态分布的一个很好的课件，非那种艰涩的纯抽象讲解，有各种具体的案例，比较适合初学者深刻理解正态分布的内涵(This is a very good courseware handouts to the normal distribution. Instead of common pure abstract explanation, there are a variety of specific cases, more suitable for beginners to understand the profound connotation of normal distribution)