已知等差数列｛an｝满足：a3=7,a5+a7 =26,an的前n项和为sn.

学习时间:2026-04-06 18:18:33 阅读：9804

已知等差数列｛an｝满足：a3=7,a5+a7 =26,an的前n项和为sn.1.求an的通项公式.2.求an的前n项之和sn.

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2026-04-06 18:18:33

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a5+a7
=a3+2d+a3+4d
=2a3+6d
2*7+6d=26
14+6d=26
6d=12
d=2
a1=7-2*2=3
an=3+2n
sn
=(a1+an)*n/2
=(3+3+2n)*n/2
=(6+2n)*n/2
=n(3+n)