等差数列的定义:首项为a1a_1a1,公差为d(d≠0)d (d eq 0)d(d≠0)的数列:a1,a1+d,a1+2d,…,a1+(n−1)d,…a_1,a_1+d,a_1+2d,…,a_1+(n-1)d,…a1,a1+d,a1+2d,…,a1+(n−1)d,… 等差数列的通项公式: an=a1+(n−1)da_n=a_1+(n-1)dan=a1+(n−1)d 等差数列的前n项和: Sn=n2[2a1+(n−1)d]=n2(a1+an)S_n=frac{n}{2}[2a_1+(n-1)d]=frac{n}{2}(a_1+a_n)Sn=2n[2a1+(n−1)d]=2n(a1+an)