Question

已知等差数列｛an｝且a1＝1 a3＝3求数列｛an｝的通项公式 求数列｛an｝前n项和sn

Answer 1

等差数列｛an｝, 且 a1＝1, a3＝3 = a1+2d = 1+2d, d = 1
通项公式 an = n, 前 n 项和 Sn = (1/2)n(n+1).

Answer 2

a1=1 a3=3,等差数列，公差=1，an=a1+(n-1)d an=n sn=na1+n(n-1)d/2