设公差为d
∵a1=3,a4+a8=26,
∴a4+a8=a1+3d+a1+7d=2a1+10d=26
可得d=2,
即得an=a1+(n-1)d=2n+1
则Sn=a1n+n(n-1)d/2=n2+2n