注意到 E(X)=Var(X)+[E(X)2=9 E(X2)=VaX)+[E(X,)]=o2+2 于是，有 E)-[2X)n) or-*] ( ) ( ) [ ( )] , ( ) ( ) [ ( )] , 2 2 2 2 2 2 2 2     = + = + = + = + E Xi Var Xi E Xi n E X Var X E X 于是，有 . ( ) 1 1 ( ) ( ) 1 1 ( ) 2 2 2 2 2 2 1 2 2      =               + − + − =      − − = = n n n n E X n E X n E S n i i 注意到