Sn=kn²+n则n>=2时,S(n-1)=k(n-1)²+(n-1)=kn²+(1-2k)n+(k-1)所以n>=2,an=Sn-S(n-1)=2kn-k+1a1=S1==k+1,符合an=2kn-k+1所以an=2kn-k+1an是等比,则a2²=a1*a3所以(3k+1)²=(k+1)(5k+1)9k²+...