有道高考真题

=n^2+cn锡

南-锡=n^2+cn a 1 = s 1 = 1

c=-1

(n-1)an-1-sn-1=(n-1)^2-(n-1)

nan-(n-1)an-1-[sn-sn-1]=

(n-1)an-(n-1)an-1 = 2(n-1)

an- an-1= 2

an = 1+(n-1)* 2 = 2n-1