原式=（9x^2 - 4y^2 - 5x^2 - 8xy + 4y^2)/4x = (4x^2 - 8xy )/4x =4x (x-2y) /4x =x-2y =2013
[(3x+2y)(3x-2y)-(x+2y)(5x-2y)]/4x=[9x^2-4y^2-5x^2-8xy+4y^2]/4x=[4x^2-8xy]/4x=x-2y=2013
