解
x-3y=0
x=3y
(4xy)/(x²-y²)-(x+y)/(x-y)
=(4xy)/(x+y)(x-y)-(x+y)²/(x-y)(x+y)
=(4xy-x²-2xy-y²)/(x-y)(x+y)
=-(x-y)²/(x-y)(x+y)
=-(x-y)/(x+y)
=-(3y-y)/(3y+y)
=-1/2
直接代入更简单
原式
=4xy/(x²-y²)-(x+y)/(x-y)
=12y²/(9y²-y²)-(3y+y)/(3y-y)
=12/8-2
=3/2-2
=-1/2