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8. Sequences and Series
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The ${4^{th}}$ term of a $G.P.$ is square of its second term, and the first term is $-3$ Determine its $7^{\text {th }}$ term.
A
$-2187$
B
$-2187$
C
$-2187$
D
$-2187$
Solution
Let $a$ be the first term and $r$ be the common ratio of the $G.P. $
$\therefore a=-3$
It is known that, $a_{n}=a r^{n-1}$
$\therefore a_{4}=a r^{3}=(-3) r^{3}$
$a_{2}=a r^{2}=(-3) r$
According to the given condition,
$(-3) r^{3}=[(-3) r]^{2}$
$\Rightarrow-3 r^{3}=9 r^{2} \Rightarrow r=-3 a_{7}=a r^{7-1}=a r^{6}=(-3)(-3)^{6}=-(3)^{7}=-2187$
Thus, the seventh term of the $G.P.$ is $-2187 .$
Standard 11
Mathematics
