Ch7-51 因而 E|∑(x-x)|=∑E(x)-E(x2 (2+2) 刀-1 O≠o 故E∑(x,-x)|=02证毕 i=1Ch7-51 ( ) ( ) 1 ( ) 1 2 1 2 1 2 E X E X n X X n E n i i n i i  = −       −  = = 因而 ( ) ( ) 2 2 2 2   =  +  − + n 1 2 2    − = n n 2 1 2 ( ) 1 1  =       − − = n i Xi X n 故 E 证毕