白球0 1 23
概率C(3,3)/C(8,3) C(3,2)C(5,1)/C(8,3)C(3,1)C(5,2)/C(8,3)C(5,3)/C(8,3)
概率1/56 15/56 30/56 10/56
所以期望
E(X)=0*1/56+1*15/56+2*30/56+3*10/56=105/56
E(X^2)=0^2*1/56+1^2*15/56+2^2*30/56+3^2*10/56=225/56
所以方差=
E(X^2)-E(X)^2
=225/56-(105/56)^2