棍丰伦与散理统针」 r-[-20+r+x 9 故D(2X3+5)=4E(X)-(E(X3)2] 2954 92 3 2 3 3 3 3 12 1 3 12 1 1 2 1 0 3 1 [ ( )] ( 2)       E X = −  +  +  +  (2 5) 3 故 D X + , 9 1 = 4[ ( ) ( ( )) ] 6 3 2 = E X − E X . 9 2954 =