因为x/y=1/3所以3x/y=1(2x^2-3xy+y^2)/(5x^2-2xy+3y^2)=(2x^2-3x/y*3xy+3x/y*y^2)/(5x^2-3x/y*2xy+3x/y*3y^2)=(2x^2-9x^2+3xy)/(5x^2-6x^2+9xy)=(2x^2-9x^2+3x/y*3xy)/(5x^2-6x^2+3x/y*9xy)=2x^2/26x^2=1/13