Sequences and Series 2 Question 20
21. Suppose that all the terms of an arithmetic progression are natural numbers. If the ratio of the sum of the first seven terms to the sum of the first eleven terms is $6: 11$ and the seventh term lies in between 130 and 140, then the common difference of this AP is
(2015 Adv.)
Show Answer
Solution:
- Given, $\frac{S _7}{S _{11}}=\frac{6}{11}$ and $130<t _7<140$
$\Rightarrow \quad \frac{\frac{7}{2}[2 a+6 d]}{\frac{11}{2}[2 a+10 d]}=\frac{6}{11} \Rightarrow \frac{7(2 a+6 d)}{(2 a+10 d)}=6$
$\Rightarrow \quad a=9 d$
Also, $\quad 130<t _7<140$
$\Rightarrow \quad 130<a+6 d<140$
$\Rightarrow \quad 130<9 d+6 d<140$
$\Rightarrow \quad 130<15 d<140$
[from Eq. (i)]
$\Rightarrow \quad \frac{26}{3}<d<\frac{28}{3} \quad$ [since, $d$ is a natural number]
$\therefore \quad d=9$
