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8. Sequences and Series
easy
The sum of a $G.P.$ with common ratio $3$ is $364$, and last term is $243$, then the number of terms is
A
$6$
B
$5$
C
$4$
D
$10$
Solution
(a) $\frac{{a{r^n} – a}}{{r – 1}} = 364$
$ \Rightarrow \frac{{a{r^{n – 1}}.r – a}}{{r – 1}} = 364 …..(i)$
==> $\frac{{3 \times 243 – a}}{2} = 364$
$ \Rightarrow a = 1$
Now, putting this in (i) , $n = 6.$
Standard 11
Mathematics