Ⅹ 0 T X n X I mIX mo T x -m IT er  − =    n r n YX m l l m    −   − =   − =    lm m l l m YX m l l m yx y x xy n r n          −   −  =  m l l m m l l m yx y r x xy r r          Xn  Yn   r  x y r  o   −   =    lm YX yx y x xy nn    