A difficult math question about arithmetic sequence?
2019-01-16 10:20 pm
Let T(n) be the n-th term of an arithmetic sequence. If T(12) - T(7) = 15 and 3T(1)=T(3), then T(21)=?
回答 (1)
2019-01-17 10:19 pm
✔ 最佳答案
∵ T(n) = a + (n - 1)d{ T(12) = a + 11d . . . . . . . . . . . ①
{ T(7) = a + 6d . . . . . . . . . . . . . ②
①-② : T(12) - T(7) = 5d
15 = 5d (given)
∴d = 3
3T(1) =T(3)
3a = a + 2d
2a = 2d
∴a = d = 3
T(21) = 3 + (21-1)(3) = 63
2019-01-16 11:56 pm
Let T(n) = a + d(n - 1)
5d = 15
d = 3
3a = a + 2d
a = 3
T(21) = a + 20d = 63 #
5d = 15
d = 3
3a = a + 2d
a = 3
T(21) = a + 20d = 63 #
收錄日期: 2021-04-24 01:15:09
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20190116142046AAYbapk