问题
解答题
已知{an}为等比数列,a1=1,a4=27.Sn为等差数列{bn}的前n项和,b1=3,S5=35.
(1)求{an}和{bn}的通项公式;
(2)设Tn=a1b1+a2b2+…+anbn,求Tn.
答案
(1)设等比数列的公比为q
∵{an}为等比数列,a1=1,a4=27,∴公比q=3,∴an=3n-1,(3分)
设等差数列{bn}的公差为d,
∵Sn为等差数列{bn}的前n项和,b1=3,S5=35,∴15+10d=35,∴d=2
∴bn=2n+1. (6分)
(2)Tn=a1b1+a2b2+…+anbn=3×1+5×3+…+(2n-1)×3n-2+(2n+1)×3n-1①
3Tn=3×3+5×32+…+(2n-1)×3n-1+(2n+1)×3n②
①-②得:-2Tn=3+2×(3+32+…+3n-1)-(2n+1)×3n(9分)
∴Tn=n•3n(12分)