(x+y)(x-3y)=0x=-y或x=3y当x=-y时(xy-2y²)/(xy+4y²)=(-y²-2y²)/(-y²+4y²)=-1当x=3y时(xy-2y²)/(xy+4y²)=(3y²-2y²)/(3y²+4y²)=1/7