6+1 E(x)=∑E(X) 6+2 D(X)=E(X)-(EX)96+10+1 6+30+2 正确求解 D()=G=D)1 EX2-(EX n n 0+1(+1) 6+1 n+3(+2)2」m(0+3+2)6 -1(3) 21 ( ) 1 ( ) 1 ++ =  = =  n i E Xi n E X 2 2 2 ) 21 ( 31 ( ) ( ) ( ) ++ − ++ = − =   D X E X E X ? 正确求解   2 2 2 ( ) ( ) 1 ( ) i i i EX EX n n D X n D X = = = −  2 2 2 ( 3)( 2) 1 ( 2) ( 1) 3 1 1 + + + =  ++ − ++ =      n n