B,=uonl B2=un wm 2lo 2 S(L-X+2SX (onI S(L-Ⅹ)+ SX o n212 S(4-A0)( ) ( ) ( ) ( ) ( ) 1 0 2 2 2 1 2 m 0 2 2 0 0 2 2 X 0 I=C B B W = S L X + S X 2 2 = S L X + S X 2 2 F 2 m B nI B nI nI nI W n I S x           = = − −  = = − 