已知等差数列(an)满足:a3=7,a5+a7=26,(an)的前n项和为Sn
2个回答
因为等差数列,a5+a7=26
所以a6-d+a6+d=26
a6=13
a3=7=a1+2d
a6=13=a1+5d
所以d=2
an=2n+1
sn=n(n+2)
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