试题难度:难度:中档 试题类型:解答题 试题内容:等差数列{an}中,a7=4,a19=2a9,
(I)求{an}的通项公式;
(II)设bn=1nan,求数列{bn}的前n项和Sn.
试题答案:(I)设等差数列{an}的公差为d∵a7=4,a19=2a9,∴a1+6d=4a1+18d=2(a1+8d)解得,a1=1,d=12∴an=1+12(n-1)=1+n2(II)∵bn=1nan=2n(n+1)=2n-2n+1∴sn=2(1-12+12-13+…+1n-1n+1)=2(1-1n+1)=2nn+1