E(X)=xf(rdx=x/ x/0 dx 2-x/6 r e b2x-x4x=26 +2 于是D(X)=E(2)[E(2=202-02=02 即有E(X)=e,D(X)=027 于是 D(X)=E(X2 )-[E(X)]2=2 2- 2= 2 . 即有 E(X)=, D(X)= 2 . e 2 e d 2 , e d 1 ( ) ( )d 2 0 / 0 2 / 0 2 2 2 / |      = - + = = =     -  -  -  - x x x E X x f x x x x x x x