a^2·a^3+(-a^2)^3-2a(a^2)^3-2［(a^3)^3÷a^3］
=a^5+(-a^6)-2a.a^6-2(a^9÷a^3)
=a^5-a^6-2a^7-2a^6
=a^5-3a^6-2a^7