设离散随机变量X的分布列为：
x-101/212
p1/31/61/61/121/4
求：(1)E(X)；
(2)E(X2)；
(3)D(X)．
(1)E(x)=(-1)×(1/3)+0×(1/6)+(1/2)×(1/6)+1×(1/12)+2×(1/4)=1/3 (2)E(X2 )=(-1)2×(1/3)+02×(1/6)+(1/2)2 ×(1/6)+12+22×(1/4)=35/24． (3)D(X)=E(C)2 =(E(X))2 =35/24-1/9 =97/72
设离散随机变量X的分布列为：
x-101/212
p1/31/61/61/121/4
求：(1)E(X)；
(2)E(X2)；
(3)D(X)．
(1)E(x)=(-1)×(1/3)+0×(1/6)+(1/2)×(1/6)+1×(1/12)+2×(1/4)=1/3 (2)E(X2 )=(-1)2×(1/3)+02×(1/6)+(1/2)2 ×(1/6)+12+22×(1/4)=35/24． (3)D(X)=E(C)2 =(E(X))2 =35/24-1/9 =97/72
