Tag: geometric sequences

Questions Related to geometric sequences

If there exists a geometric progression containing 27, 8 and 12 as three of its terms (not necessarily consecutive) then no. of progressions possible are

  1. $1$

  2. $2$

  3. infinite

  4. None of these


Correct Option: C
Explanation:
$8, 12, 29$

Let the first term be $8$

$\therefore a = 8$    ...(i)

Let the $p^{th}$ ther be $12$

$\therefore ar^{p-1} = 12$     ...(ii)

and $q^{th}$ term be $27$

$\therefore ar^{q-1} = 27$    ...(iii)

(i) / (ii)

$\dfrac{a}{ar^{p-1}} = \dfrac{8}{12}$

$\Rightarrow r^{p-1} = \dfrac{3}{2} = 1.5$      ...(iv)

(iii) / (i)

$r^{q-1} = \dfrac{27}{8} = \left(\dfrac{3}{2}\right)^3$

$r^{q-1} = (1.5)^3$    ...(v)

(v) / (iv)

$\Rightarrow \dfrac{r^{q-1}}{r^{p-1}} = \dfrac{(1.5)^3}{1.5}$

$\Rightarrow r^{q-1-p+1} = (1.5)^2$

$\Rightarrow r^{q-p} = (1.5)^2$

              $= (r^{p-1})^2$     ...from (iv)

$\Rightarrow r^{q-p} = r^{2p-2}$

$\therefore q-p = 2p-2$

$\Rightarrow \boxed{q=3p-2}$

for every distinct value of '$p$' there will be a district integer value of '$q$'

$\therefore $ Infinite no. of progression are possible.

option $C$