3A^2-A(A-5B)-B(2A-B)^2
=3A^2-A^2+5AB-B(4A^2-4AB+B^2)
=3A^2-A^2+5AB-4A^2B+4AB^2-B^3
=2A^2-4A^2B+4AB^2-B^3+5AB
{a^b表示a的b次方}