1.

This kind of recursive sequence can get its general term formula by accumulation (sequence {f(n)} can get the sum of the first n terms).

When it is a constant, the general term formula of arithmetic progression can be obtained by the accumulation method. When it is arithmetic progression, it is a second-order arithmetic progression, and its general term formula should be formal. Note the difference between it and the sum formula of arithmetic sequence in general form, that is, its constant term must be 0.

2.

This kind of recursive sequence can get its general term formula by cumulative multiplication (sequence {g(n)} can get the product of the first n terms).

When it is a constant, the general term formula of geometric series can be obtained by multiplication.

3.；

This kind of sequence can usually be transformed into, or the elimination constant can be transformed into a second-order recursive formula.