Sequences and Series (001)

问题描述：

Sequences and Series (001)
If the first,fifth and tenth terms of an arithmetic progression are in geometric progression and the sum of the second and eighth terms is 20,find the first term and the (non-zero) common difference.

1个回答分类：英语 2014-11-25

问题解答：

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Solution:let the geometric progression a、a+4d and a+9d.
∴q^2=(1+4d/a)^2=1+9d/a.
∴d/a=1/16,or d/a=-1/16.
When d/a=-1/16,it is not to fit this question .
So d/a=1/16.
∴ a=16d.
∵(a+q)+(a+7q)=20
∴a+4d=10
∵20d=10
∴d=1/2
∴ a=8

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