这是离散型分布：期望值E（X）=0.5*P(X=1100)+0.5*P(X=900)=1000

方差D(X)=E(X^2)-[E(X)]^2

=1010000-1000^2=10000

标准差s=根号D（X）=100标准差/期望值=100/1000=1/10