X 0 1 2 3 Px(x;) 9 39 329 3219 12 1211 121110 1211109 Px(x;) 9/12 9/44 9/220 1/220 E(X)=0x3+1x 9 9 1 +2× +3× 0.3； 4 44 220 220 E(X2)=0x3+1× 9 1 +4× +9× =0.4; 44 220 220 D(X)=E(X2)-IE(X)12=0.31.( ) ( ) [ ( )] . . ( ) . ; ( ) . ; 0 31 0 4 220 1 9 220 9 4 44 9 1 4 3 0 0 3 220 1 3 220 9 2 44 9 1 4 3 0 2 2 2 = − = =  +  +  +  = =  +  +  +  = D X E X E X E X E X 9/12 9/44 9/220 1/220 X 0 1 2 3 p ( x ) X i p ( x ) X i 12 9 11 9 12 3  10 9 11 2 12 3   9 9 10 1 11 2 12 3   