a^5+2a^3-a-2=（a^5-a）+(2a^3-2)
=a(a^4-1)+2(a^3-1)
=a(a^2+1)(a^2-1)+2(a-1)(a^2+1+a)
=a(a^2+1)(a+1)(a-1)+2(a-1)(a^2+1+a)
然后提公因式（a-1）；然后我相信你就会做了.