x分之1-y分之1=41/x - 1/y = 4y - x = 4xyxy = y/4 - x/4,代入 (2x-xy-2y)/（x+3xy-y）,得：(2x - y/4 + x/4 - 2y)/（x + 3y/4 - 3x/4 - y）= (9x/4 - 9y/4)/ (x/4 - y/4)= 9