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

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)