若某等差數列首25項的總和是-1450,公差是-9,求數列的首項
求等差-46,-34,-22....,326的總和
maths數學等差問題???
2010-10-28 7:02 pm
回答 (1)
2010-10-28 7:16 pm
✔ 最佳答案
設首項為a(25/2)[2a+(25-1)(-9)]=-1450
2a+(24)(-9)=-1450x2/25=-116
2a=-116+216
2a=100
a=50
-46, -34, -22, ...326
T(n)=-46+(n-1)(12)=-46+12n-12=-58+12n
326=-58+12n
384=12n
n=32
S(n)
=(-46+326)x32/2=4480
收錄日期: 2021-04-25 12:45:37
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20101028000051KK00308