1,在自然数从小到大排成的数列中,前n个奇数之和Sn等于多少?2.在等差数列﹛an﹜中,S19=102,则a10等于多少?（ps:答案我知道,

Question

1,在自然数从小到大排成的数列中,前n个奇数之和Sn等于多少?
2.在等差数列﹛an﹜中,S19=102,则a10等于多少?
（ps:答案我知道,

Answer 1

(1)1,3,5.2n-1 等差数列
An= 2n-1
Sn=(a1+an)n/2 = (1+2n-1)n/2 =n^2
(2) 因为a1+a19 = 2a10,a2+a18= 2a10,a3+a17 = 2a10,...a9+a11 = 2a10,
S19=19a10
a10= S19/19 =102/19