先简化,再代入,（x-y）/(x-3y)=2/7 ,另外一个种,直接代入计算详细一点。。。x/2=y/3,那么y=3x/2 ,代入方程式，可以得出2/7（x^2-y^2）/(x^2-2xy-3y^2)=[(x+y)(x-y)]/[(x-y)^2-4y^2]=[(x+y)(x-y)]/[(x-y)^2-(2y)^2]=[(x+y)(x-y)]/[(x-y-2y)(x-y+2y)]=[(x+y)(x-y)]/[(x-3y)(x+y)]=(x-y)/(x-3y)y=3x/2 代入 (x-y)/(x-3y)=（x-3x/2）/(x-9x/2)=2/7或者可以用x=2y/3 代入，结果一样