设数列 {an} 的前 n 项和为 Sn，则 {an} 等差。
(1) S n4=n 2+2n, n=1,2,3
(2) S n4=n 2+ 2n +1,n=1,2,3
A
等差数列的前n项和公式为Sn=An2+Bn，A、B为全体实数，因此条件(1)充分，条件(2)不充分，选A。
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