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-1

3Tn=3×3+5×32+…+(2n-1)×3n-1+(2n+1)×3n

①-②得:-2Tn=3+2×(3+32+…+3n-1)-(2n+1)×3n(9分)

Tn=n•3n(12分)

判断题
单项选择题 A1型题