§2.2 等差数列 1.等差数列的判定 (1)an-an-1=d (n≥2,d为常数)⇔{an}是公差为d的等差数列; (2)2an=an-1+an+1 (n≥2)⇔{an}是等差数列; (3)an=kn+b(k,b为常数)⇔{an}是公差为k的等差数列(n≥1); (4)sn=an2+bn(a,b为常数)⇔{an}是公差为2a的等差数列(n≥1).例如:已知等差数列{an}的前n项和sn=(n-1)2+λ,则λ的值是________.解析 sn=(n-1)2+λ=n2-2n+(1+λ), ∵{an}是等差数列,∴1+λ=0,λ=-1. 答案 -1