An = a 1+2a2+3a3+...+(n- 1) an- 1。

1 minus 2

An-an-1= (n-1) an-1,which means An=nAn- 1.

A2=(2- 1)A 1

This part is brought in directly. Note that the original title is n & gt=2, and if you accidentally A2=A 1+2A2, there will be a negative sign.

a2 = a 1 = 1 th EN A3 = a 1+2 a2 = 3

The general formula of branch one {An} is n! /2