(x-3y)(y+3x)-(x-3y)(3y-x)
=（x-3y)(y+3x-(3y-x))
=(x-3y)(y+3x-3y+x)
=(x-3y)(4x-2y)
=2(2x-y)(x-3y)��(��999�����֮��)²���ڶ���(﹣999又三分之二)²=(999又三分之二)²=(999+2/3)^2=(999+1-1+2/3)^2=(1000-1/3)^2=1000000-2/3*1000+1/9