Question:
The sum of the first 7 terms of an AP is 182. If its 4th and 17th terms are in the ratio 1 : 5, find the AP.
Solution:
Let a be the first term and d be the common difference of the AP.
$\therefore S_{7}=182$
$\Rightarrow \frac{7}{2}(2 a+6 d)=182 \quad\left\{S_{n}=\frac{n}{2}[2 a+(n-1) d]\right\}$
$\Rightarrow a+3 d=26 \quad \ldots(1)$
Also,
$a_{4}: a_{17}=1: 5$ (Given)
$\Rightarrow \frac{a+3 d}{a+16 d}=\frac{1}{5} \quad\left[a_{n}=a+(n-1) d\right]$
$\Rightarrow 5 a+15 d=a+16 d$
$\Rightarrow d=4 a$ $\cdots \cdots(2)$
Solving (1) and (2), we get
$a+3 \times 4 a=26$
$\Rightarrow 13 a=26$
$\Rightarrow a=2$
Putting a = 2 in (2), we get
$d=4 \times 2=8$
Hence, the required AP is 2, 10, 18, 26, ... .