由(1)因式分(x+y)²-(x+y)³
=(x+y)²(1-x-y)
或者(2)计算:(x+y)²-(x+y)³
=x²+2xy+y²-x²=³-3x²y-3xy²-y³.
(x+y)的二次方减(x+y)的三次方
=x²+2xy+y²-x²=³-3x²y-3xy²-y³。