Rút gọn biểu thức
3(x-y)^2-2(x+y)^2-(x-y)(x+y)
$=(x-y)^2+2(x-y)^2-2(x+y)^2-(x-y)(x+y)\\=[(x-y)^2-(x-y)(x+y)]+[2(x-y)^2-2(x+y)^2]\\=(x-y)(x-y-x-y)+2[(x-y)^2-(x+y)^2]\\=(x-y)(-2y)+2(x-y+x+y)(x-y-x-y)\\=(x-y)(-2y)+2(2x)(-2y)\\=(-2y)(x-y+4x)\\=-2y(5x-y)$