Between 50 and 400, find
（a) the sum of all the integers with units digit equal to 3
(b) the sum of all the integers with units digit equal to 2 or 7
中一 maths 20分?
2020-07-26 7:28 pm
回答 (1)
2020-07-26 10:06 pm
✔ 最佳答案
(a)a(n) = 10n+3, n = 5,6,...,39Σ{n=5～39} a(n) = Σ{n=5～39} (10n+3) = 10 Σ{n=5～39} n + Σ{n=5～39} 3
= 10 (5+39)(35)/2 + 3(35)
= 7700 + 105
= 7805
(b)
b(n) = 10n+2, n = 5,6,...,39
c(n) = 10n+7, n = 5,6,...,39
Σ{n=5～39} (b(n)+c(n))
= Σ{n=5～39} b(n) + Σ{n=5～39} c(n)
= [7700+2(35)] + [7700+7(35)]
= 7770 + 7945
= 15715
收錄日期: 2021-05-03 20:36:23
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20200726112833AAB4N4N