2-15 d(x)=a1+ X 2 X fAx d(△)= E(1+2,1x) 4F7 △Z=d(△O) nEd,d2-15 1 2 0 2 1 2 1 2 1 2 1 2 1 1 4 ( ) ( ) 4 ( ) ( ) 4 ( ) ( ) Ed d Fl l d l x l d d E d Fdx d l x l d d A x d x l d d d x d l     =  = − +  = − = + − = + 