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JAMB Mathematics 2019 Paper

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Question : 60

Total: 73

If the 3rd and 7th terms of a G.P are 9 and 1/9 respectively. Find the common ratio.

$\frac{1}{3}$
$\frac{1}{9}$
$\frac{2}{3}$
$\frac{2}{9}$

Solution:

$T_n = ar^{n-1}$ (terms of a G.P)

$T_3 = ar^2 = 9$ ... (i)

$T_7 = ar^6 = \frac{1}{9}$ ... (ii)

Divide (i) by (ii);

$\frac{ar^6}{ar^2} = \frac{\frac{1}{9}}{9}$

$r^4 = \frac{1}{81}$

$r^4 = \left(\frac{1}{3}\right)^4$

$r = \frac{1}{3}$

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