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

×
3

×
4

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5

×
6
How many terms of the series 1 + 3+ 9+…. sum to 121?
3
4
5
6
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^n1}{31}
\therefore n =5