E[-∑X1-2X2+X2] =E[-∑X1-X2) ∑E(X12)-E(X2)2 ] 1 [ 2 2 2 X X X n = E  i − + ) 1 [ 2 2 X X n = E  i − ( ) ( ) 1 2 2 E X E X n =  i −