仿P:191~-192求得E(S2)<2 a2与题无关,等于没求! 正确求解 6+1 E(S2)=0=D(X) (0+3)(+2)2 6-2(1) 9 CNP(1-p) k=0.1….N x怎能取两项概率?2 2 2 ( 3)( 2) 1 ( ) ( ) + + + = = =     D Xi E S 仿P.191~192 求得 2 2 E(S ) = 2  与题无关，等于没求！ 正确求解 6-2 (1) ( )         = = −  = − k N x C p p x x k k N k i N n 0,1, , (1 ) , , , 1   ? xi 怎能取两项概率?