t^3-2t[t^2-2(t-3)]=t^3-2t[t^2-2t+6]=t^3-2t^3+4t^2-12t=-t^3+4t^2-12t选A
t^3-2t[t^2-2(t-3)]=t^3-2t[t^2-2t+6]=t^3-2t^3+4t^2-12t=-t^3+4t^2-12t
