x/x-y=a
取倒数
(x-y)/x=1/a
1-(y/x)=1/a
(2x^2y-2xy^2)/3x^2y
上下同时除以x²y
=(2-2(y/x))/3
=(2/3)(1-(y/x))
=(2/3)(1/a)
=2/(3a)