Question

1、已知两个等差数列{An}和{Bn}的前n项和之比为7n+1/4n+27 （n属于正整数）,则a11/b11等于 （

Answer 1

1)C

An=a1*n+n(n-1)*D/2

Bn=b1*n+n(n-1)*d/2

则An/Bn=(a1*n+n(n-1)*D/2)/(b1*n+n(n-1)*d/2)=(a1-D/2+n*D/2)/(b1-d/2+n*d/2)

所以可对应得a1=8,D=14 b1=31,d=8

所以a11/b11=(8+14*10)/(31+8*10)=4/3

2)D

S1=a1 则a1=3a1/2-3 可解得a1=6

S2=a1+a2 则6+a2=3a2/2-3可解得a2=18

此时可以用排除法,选的答案为D