英语翻译1 the tenth term of an arithmetic series is 67 and the s

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英语翻译
1 the tenth term of an arithmetic series is 67 and the sum of the first twenty terms is 1280.find the common difference ,d and the first term a .
2 if the first term of an arithmetic sequence is 3 and the hundredth tern is 307,then find the sum of the first 100 terms.
3 an arithmetic sequence has thirteen terms whose sum is 143.the third term is 5.find the first term

1、一个等差数列的第10项是67,前20项和是1280.求该数列的公差和首项.
可以设出首项和公差,解一个二元一次方程组,得到首项为121,公差为-6
2、设一个等差数列的首项为3,第100项为307,试求出前100项和
使用等差数列的前n项和公式,算得15500
3、设一个等差数列有13项,其和为143,第五项为5,求其首项
由前13项和为143,可知第七项为11,又由第五项为5,进而求出公差为3,继而
求出首项为-7

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