+)=xm EH2-mE2=m(a2+2)-a2-n2 (n-1) 其中EX2=DX+(EX)2=a2+12 EX= DX+(EX)9 即 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 2 2 1 2 1 2 ( )( ) ( 1) ( ) ( ) m s s m s s m s m = + = + = + = + = − = − = + − −   = −   −   = = =n EX DX EX EX DX EX n EX nEX n n E X X E X nX i i i ni i ni i ni i 其中