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。
Successful learning is not a(n)________activity but consists of four distinct stages in a specific order
Inordertolivethekindoflifewewantandtobethepersonwewanttobe，wehavetodomorethanjust________withevents．
Itisestimatedthat，currently,about50，000speciesbecome_____everyyear．
Tom placed the bank notes ，_________the change and receipts ，back in the drawer ．
About20ofthepassengerswhowereinjuredinaplanecrasharesaidtobein_____condition．