(a)find 51+52+53+..............+100
(b)find 21+22+23+....+50
更新1:
我要條式呀-3-
two easy maths!!
2008-10-11 9:10 pm
given that the sum of the integers from 1 to n is n(n+1)/2
(a)find 51+52+53+..............+100
(b)find 21+22+23+....+50
(a)find 51+52+53+..............+100
(b)find 21+22+23+....+50
回答 (4)
2008-10-11 9:47 pm
✔ 最佳答案
1 + 2 + 3 + ...... + n = n(n + 1)/2
(a)
51 + 52 + 53 + ...... + 100
= [1 + 2 + 3 + ...... + 100] - [1 + 2 + 3 + ...... + 50]
= [100(100 + 1)/2] - [50(50 + 1)/2]
= 5050 - 1275
= 3775
=====
(b)
21 + 22 + 23 + ...... + 50
= [1 + 2 + 3 + ...... + 50] - [1 + 2 + 3 + ...... 20]
= [50(50 + 1)/2] - [20/(20 + 1)/2]
= 1275 - 210
= 1065
=
2008-10-12 2:20 am
a)
(1+100)100/2-(1+50)50/2
=5050-1275
=3775
b)
(1+50)50/2-(1+20)20/2
=1275-210
=1065
(1+100)100/2-(1+50)50/2
=5050-1275
=3775
b)
(1+50)50/2-(1+20)20/2
=1275-210
=1065
2008-10-12 12:43 am
a:(51+100)x50÷2
151x50÷2
151x25
=3775
b：(21+50)x30÷2
71x30÷2
71x15
=1065
2008-10-11 16:54:05 補充：
答案得來的方法是:
(頭項+未項)x項數÷2
( 51 + 100) x 50 ÷ 2
151x50÷2
151x25
=3775
項數得來的方法是:
(未項-頭項)+1
( 100 - 51 )+1
=49+1
=50
151x50÷2
151x25
=3775
b：(21+50)x30÷2
71x30÷2
71x15
=1065
2008-10-11 16:54:05 補充：
答案得來的方法是:
(頭項+未項)x項數÷2
( 51 + 100) x 50 ÷ 2
151x50÷2
151x25
=3775
項數得來的方法是:
(未項-頭項)+1
( 100 - 51 )+1
=49+1
=50
參考: me
2008-10-11 9:16 pm
a = 3775
b = 1065
b = 1065
收錄日期: 2021-04-19 02:06:03
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20081011000051KK00924