Question

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

• A. $5$
• B. $6$
• C. $4$
• D. $3$

Answer 1

The correct option is A $5$

The given series is a G.P
Sum of n terms of a G.P is $\left(\frac{a(r^n-1)}{(r-1)}\right)$
Here $a=1$ and $r=3$
Substituting it in the equation and finding $n$ we get $n=5.$