a1,a3,a6成等比数列
a3²=a1a6
(a1+2d)²=a1(a1+5d)
a1²+4a1d+4d²=a1²+5a1d
a1d=4d²
d≠0
d=a1/4=1/2
所以an=a1+(n-1)d=n/2+3/2
所以Sn=(a1+an)n/2=(n²+7n)/4
a1,a3,a6成等比数列
a3²=a1a6
(a1+2d)²=a1(a1+5d)
a1²+4a1d+4d²=a1²+5a1d
a1d=4d²
d≠0
d=a1/4=1/2
所以an=a1+(n-1)d=n/2+3/2
所以Sn=(a1+an)n/2=(n²+7n)/4