用方差的定义：
D(X)=(1/18)[2(34.5-54.5)^2+3(44.5-54.5)^2+7(54.5-54.5)^2+5(64.5-54.5)^2+(74.5-54.5)^2]
=(1/18)[800+300+0+500+400)
=1000/9
=111.1.
下面是一个公司雇员年龄的频率分布.
用方差的定义：
D(X)=(1/18)[2(34.5-54.5)^2+3(44.5-54.5)^2+7(54.5-54.5)^2+5(64.5-54.5)^2+(74.5-54.5)^2]
=(1/18)[800+300+0+500+400)
=1000/9
=111.1.