Question

How many terms of the series 1+3+9+......., sum to 1093?

• A. 5
• B. 6
• C. 7
• D. 3

Answer 1

Answer: (C)

7

the given series is a G.P
sum of $n$ terms of a G.P is $a\ast \left(\frac{(r^n-1)}{(r-1)}\right)$
here $a=1$
and $r=3$
So, $1\ast \left(\frac{(3^n-1)}{(3-1)}\right)=1093$
Thus, $\left(\frac{(3^n-1)}{2}\right)=1093$

$3^n=2187$

We get $n=7$