二項分布(binomial distribution)

$$B(n,p)=f_X(x)=\{\begin{array}{ll}{\displaystyle {}_n{C}_x{p}^x(1-p{)}^{n-x}}& x\in \{0,1,2,\dots ,n\}\\ {\displaystyle 0}& x\notin \{0,1,2,\dots ,n\}\end{array}$$