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已知等差数列{an}中s10=100,s20=300,则s30=
人气:321 ℃ 时间:2020-05-10 05:30:16
解答
设s30为x,由题意可得
(s20-s10)/s10=(s30-s20)/(s20-s10)
即(300-100)/100=(x-300)/(300-100)
200^2=100(x-300)
解得x=700
所以s30=700
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