(am-bm)-(an-bn)
(2x^4-x^3)-(3x^2y-y^2)
(3x-4y+2xy) - (2x-5y+5xy)=3x-4y+2xy-2x+5y-5xy
=x+y-3xy
=3-3X(-2)
=3+6=9
去括号》（只有三题,只是我笨.）
(am-bm)-(an-bn)
(2x^4-x^3)-(3x^2y-y^2)
(3x-4y+2xy) - (2x-5y+5xy)=3x-4y+2xy-2x+5y-5xy
=x+y-3xy
=3-3X(-2)
=3+6=9