X²+3XY-4Y²=0
(X+4Y)(X-Y)=0
X=-4Y,X=Y
若X=-4Y,(X-Y)/(X+Y)=(-4Y-Y)/(-4Y+Y)=-5Y/(-3Y)=5/3
若X=Y,(X-Y)/(X+Y)=(Y-Y)/(Y+Y)=0