8. Sequences and Series
medium

$3$ અને $81$ વચ્ચે બે સંખ્યામાં ઉમેરો કે જેથી બનતી શ્રેણી સમગુણોત્તર હોય.

A

$9,27$

B

$9,27$

C

$9,27$

D

$9,27$

Solution

Let $G_{1}$ and $G_{2}$ be two numbers between $3$ and $81$ such that the series, $3, G_{1}, G_{2}, 81,$ forms a $G.P.$

Let $a$ be the first term and $r$ be the common ratio of the $G.P.$

$\therefore 81=(3)(r)^{3}$

$\Rightarrow r^{3}=27$

$\therefore r=3$ (Talking real roots only)

For $r=3$

$G_{1}=a r=(3)(3)=9$

$G_{2}=a r^{2}=(3)(3)^{2}=27$

Thus, the required two numbers are $9$ and $27$

Standard 11
Mathematics

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