a2-a 1=3

a3-a2=3

a4-a3=3

……

an-an- 1=3

The left and right of the above items are added respectively, and an-a 1=3(n- 1) is obtained after positive and negative elimination.

∴an=3(n- 1)+a 1=3n-2.

The so-called superposition method refers to the process of adding the left and right formulas in the middle, removing the middle and leaving only the head and tail.