Question:
The $9^{\text {th }}$ term of an A.P. is equal to 6 times its second term. If its $5^{\text {th }}$ term is 22 , find the A.P.
Solution:
Let a be the first term and d be the common difference.
We know that, nth term = an = a + (n − 1)d
According to the question,
a9 = 6a2
⇒ a + (9 − 1)d = 6(a + (2 − 1)d)
⇒ a + 8d = 6a + 6d
⇒ 8d − 6d = 6a − a
⇒ 2d = 5a
$\Rightarrow a=\frac{2}{5} d$
Also, a5 = 22
⇒ a + (5 − 1)d = 22
⇒ a + 4d = 22 ....(2)
On substituting the values of (1) in (2), we get
$\frac{2}{5} d+4 d=22$
$\Rightarrow 2 d+20 d=22 \times 5$
$\Rightarrow 22 d=110$
⇒ d = 5
$\Rightarrow a=\frac{2}{5} \times 5 \quad[$ From (1) $]$
⇒ a = 2
Thus, the A.P. is 2, 7, 12, 17, .... .