9x³y-7x²y²+2xy³
=xy(9x²-7xy+2y²)
不能再分解下去了
若上面2xy³前面是-的话可以继续分
9x³y-7x²y²-2xy³
=xy(9x²-7xy-2y²)=xy(x-y)(9x+2y)