证明：设从总体中抽两组样本：X1,X2,X1,; X:X=是2XX=2X s=2x,-X八9=m2x-x m-w空，之x 西a空+空 -4X1+n,X2 n1+n2. ( ) ( ) ( ) ( ) , ( ) . , ,., ; , , , ,., ; 1 2 1 1 2 2 1 2 2 2 1 1 1 1 1 2 1 2 1 1 1 2 2 2 1 2 2 2 2 2 1 1 1 1 2 1 1 2 2 2 1 1 1 2 1 2 2 2 1 1 1 1 2 1 1 2 1 2 1 2 1 2 2 1 1 1 1 1 1 1 1 1 1 1 1 n n n X n X X n X n n n n n X X n n X X X n X X S n S X n X X n X X X X X X X n i i n i i n i i n i i n i i n i i n i i n i n i n + + =  +  + = + + = − − − = − = = =         = = = = = = = = 证明：设从总体中抽取两组样本：