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、条件（1）充分，但条件（2）不充分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。
Itisestimatedthat，currently,about50，000speciesbecome_____everyyear．
Thepoorcountriesareextremely_______tointernationaleconomicfluctuations
Allherhardwork__________intheend，andshefinallypassedtheexam．
Afterhisuncledied，theyoungman_____thebeautifulestatewithwhichhechangedfromapoormantoawealthynoble．
TheinteractionsbetweenChinaandtheUSwillsurelyhaveasignificant_______on.peaceandstabilityintheAsia.—Pacificregionandtheworldasawhole.
