x/y=1/3
(2x^2-3xy+y^2)/(5x^2-2xy+3y^2)
分子分母同时除以y^2
={2(x/y)^2-3x/y+1}/{5(x/y)^2-2x/y+3}
=(2/9-1+1)/(5/9-2/3+3)
=1/13