英文数列题目 In a particular arichmetic progression,the sum of the 9th and 10th terms is-64.AlsoIn a particular arichmetic progression,the sum of the 9th and 10th terms is-64.Also,the difference between the sum of the first 5 terms and 2 times
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英文数列题目 In a particular arichmetic progression,the sum of the 9th and 10th terms is-64.AlsoIn a particular arichmetic progression,the sum of the 9th and 10th terms is-64.Also,the difference between the sum of the first 5 terms and 2 times
英文数列题目 In a particular arichmetic progression,the sum of the 9th and 10th terms is-64.Also
In a particular arichmetic progression,the sum of the 9th and 10th terms is-64.Also,the difference between the sum of the first 5 terms and 2 times the fifth term is-49.What is the sum of the first 30 terms?
英文数列题目 In a particular arichmetic progression,the sum of the 9th and 10th terms is-64.AlsoIn a particular arichmetic progression,the sum of the 9th and 10th terms is-64.Also,the difference between the sum of the first 5 terms and 2 times
先翻译一下吧,在一个等差数列中,第九项和第十项的和是-64,前五项的和与第五项的两倍数之间相差-49,求前三十项的和.
作为文科生表示压力很大~但还是解一下吧~首项为-15,公差是-2,前三十项的和是-1320,
a9+a10=-64 ------2a1+17d=-64
a1+a2+a3+a4+a5 - 2a5 =-49
3a1+2d=-49
联合两条式子求出a1 和 d 就可以了