21(a-b)³-35(b-a)²=
=7(a-b)²(3a-3b-5)
4a(x+y-z)+10a²(z-x-y)
=2a(x+y-z)(2-5a)
已知:ab=1,a+b=-2007,
则a²b+ab²=ab(a+b)=-2007
6(x-y)³+15(y-x)²-9(y-x)³
=15(x-y)³+15(x-y)²
=15(x-y)²(x-y+1)