(4x^2-8xy+4y^2)/(2x^2-2y^2)
=4(x²-2xy+y²)/2(x²-y²)
=4(x-y)²/2(x+y)(x-y)
=2(x-y)/(x+y)
把x=3y代入上式得
原式=2(3y-y)/(3y+y)=1