答：
x²+3xy-4y²=0
(x+4y)(x-y)=0
所以x=-4y,或x=y
当x=-4y时,(x-y)/(x+y)=-5y/-3y=5/3
当x=y时,(x-y)/(x+y)=0
所以(x-y)/(x+y)=0或5/3