The negative binomal distribution(负二项分布)
https://statisticalmodeling.wordpress.com/2011/07/28/the-negative-binomial-distribution/
total variance(全方差)
https://zhuanlan.zhihu.com/p/507900722
moment generate function(矩生成函数)
like life ,when it's complicated ,just simplifying it ,grasping it and understanding it ,by appreciate the moments.
the statement gives us a simple and clear meaning of moment(矩),just simplifying the complicated probability distribution and let us use some moment informations to describe the distribution