p=1-0.3-0.5
=0.2
E（X）=0*0.3+1*0.2+2*0.5
=1.2
E（X-1）也就等于：
E（x）-E（1）
=E（x）-1
=1.2-1
=0.2
D（X-1)
=E[(X-1)^2]-[E(X-1)]^2
=0.8-0.2^2
=0.4