:(3a^n+2+6a^n+1-9a^n)÷3a^n-1
=3a^n+2÷3a^n-1+6a^n+1÷3a^n-1-9a^n ÷3a^n-1
=a³+2a²-3a(3a^(n+2)+6a^(n+1)-9^(n))÷3a^(n-1)�����ľ������������һ��a(3a^(n+2)+6a^(n+1)-9 a ^(n))��3a^(n-1)=3a^(n+2)��3a^(n-1)+6a^(n+1)��3a^(n-1)-9a^n ��3a^(n-1)=a³+2a²-3a