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5,10,17,26通项公式
人气:194 ℃ 时间:2020-04-03 11:00:12
解答
5 =5
10 =5+5
17 =5+5+7
26 =5+5+7+9
an=5+[5+……+(2n+1)]
=5+(5+2n+1)(n-1)/2
=5+(n+3)(n-1)
=n²+2n+2
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