等差級數答案

2012-02-12 9:07 am
5+1+(-3)....+(-75)

項數跟和

拜託了~

回答 (2)

2012-02-12 9:21 am
✔ 最佳答案
首項: a1 = a = 5
公差: d = 1 - 5 = (-3) -1 = -4

an :
a + (n - 1)d = -75
5 + (n - 1)(-4) = -75
-4(n - 1) = -80
n - 1 = 20
n = 21
項數 = 21

Sn
= 5 + 1 + (-3) + .... + (-75)
= n [a1 + an] / 2
= 21 x [5 + (-75)] / 2
= -735
參考: micatkie
2012-02-12 8:29 pm
我提供不同的解法,比較不用背公式
(不過,我承認公式還是該理解而記起來的)

圖片參考:http://imgcld.yimg.com/8/n/AD01624620/o/161202120056013872760700.jpg

answered by Kaxa man

圖片參考:http://imgcld.yimg.com/8/n/AD01624620/o/161202120056013872760701.jpg
參考: kaxa's brain


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