设随机变量X的分布律P{X=K}=C/K!,K=0,1,2.则X的平方的期望是多少?

黄贞蕴回答：

　　1=Sum(k=0->无穷大)C/k!=C*Sum(k=0->无穷大)[1/k!]=C*e,C=1/e.E[x^2]=C*Sum(k=0->无穷大)k^2/k!=C*Sum(k=1->无穷大)k^2/k!=C*Sum(k=1->无穷大)k/(k-1)!=C*Sum(k=1->无穷大)(k-1+1)/(k-1)!=C*Sum(k=1->无穷大)(k-1)...