解析:∵等差数列,设首项=a1,公差=d,
∴2=4a1+6d
-6=9a1+36d
解得a1=6/5,d=-7/15,
∴Sn=na1+n(n-1)/2*d
=6n/5-7n(n-1)/30
=(43n-7n^2)/30