a(n+1) -an =2n
an -a(n-1) = 2(n-1)
an - a1 = 2( 1+2+...+(n-1))
= (n-1)n
an = n^2-n +3
an/n = (n^2-n +3)/n你好，第三步an-a（n-1）是怎么来的哦，错了，是an-a1是怎么来的，不好意思m+1=n


a(n+1) -an =2n
a(m+1) -am = 2m
a(n+1) -an =2n
an -a(n-1) = 2(n-1)