Question

7.已知E（X）=-1，D（X）=3，则E[3（X�0�5-2）]=（）。A.9；B.6；C.30；D.36。并给出详细的计算过程。

Answer 1

D(X)=E(X^2) -[E(X)]^2
3 = E(X^2) -1
E(X^2) = 4
------------
E[3（X^2-2）]
=3E(X^2) - 6
=12-6
=6
ans : B

Answer 2

D(X)=E(X^2)-[E(X)]^2E(X^2)=D(X)+[E(X)]^2=3+(-1)^2=4E[3(X^2-2)]=3E(X^2-2)=3E(X^2)-3E(2)=3*4-3*2=6