泊松分布P(X=k) = (λ^k/k!)e^(-λ) ; k=0,1,2,3,....P(X=0)+P(X=1)+..... =1--------------------------c( 3^0/0!+ 3^1/1!+....) = 1λ=3=> c = e^(-λ) =e^(-3)
