How many terms of the series 1 + 3+ 9+…. sum to 121?

Question

Uniteasy Admin · Accepted Answer

The given series is a geometric sequence with
$$ a_1 =1 $$ , and $$ r=3 $$

Sum of $$n $$ terms of a geometric sequence is
$$ S_n =  a_1 * (r^n – 1)/ (r – 1) $$
$$ S_n =a_1 \dfrac{r^n – 1}{r – 1}  $$

Therefore,

$$ 121 = \dfrac{3^n-1}{3-1}  $$

$$ 	herefore  n =5 $$

Multiple Choice Question (MCQ)

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