Question

Find S5 for a geometric series for which a1=81 and r=1/9.

Answer 1

ANSWER

$S_5=91\frac{10}{81}$

EXPLANATION

The sum of the first $n$ terms of a geometric sequence is given by;

$S_n=\frac{a_1(1-r^n)}{1-r} ,-1$

Where $n$, is the number of terms and $a_1$ is the first term.

When $n=5$, we have $a_1=81$, we get;

$S_5=\frac{81(1-(\frac{1}{9})^5)}{1-\frac{1}{9}}$

$S_5=\frac{81(1-\frac{1}{59049})}{1-\frac{1}{9}}$

$S_5=\frac{81(\frac{59048}{59049})}{\frac{8}{9}}$

$S_5=\frac{7381}{81}$

$S_5=91\frac{10}{81}$