等差数列{a n }中首项为a 1 ,公差为d,前n项和为S n ,给出下列四个命题: ①数列 { ( 1 2 ) a n } 为等比数列; ②若a 10 =3,S 7 =-7,则S 13 =13; ③ S n =n a n - n(n-1) 2 d ; ④若d>0,则S n 一定有最大值. 其中正确命题的序号是 ______.
【简答题】任务型阅读。阅读短文,按各题的任务要求答题。 The Dragon Boat Festival (端午节) is the day to remember a Chinese national hero, (1) Qu Yuan, who threw himself into the river and died over 2,000 years ago. Old story has it that...