5(x+y)(x-y)-2(x+y)^2+3(x-y)^2
=3(x-y)^2+5(x+y)(x-y)-2(x+y)^2
=(3(x-y)-(x+y))(3(x-y)+2(x+y))
=(2x-4y)(5x-y)
其中x=2,y=1
上式=(2*2-4*1)(5*2-1)
=0