Question

What is the sum of the first 5 terms of geometric series with a1=10 and r=1/5

Answer 1

$\bf \qquad \qquad \textit{sum of a finite geometric sequence} \\\\ S_n=\sum\limits_{i=1}^{n}\ a_1\cdot r^{i-1}\implies S_n=a_1\left( \cfrac{1-r^n}{1-r} \right)\quad \begin{cases} n=n^{th}\ term\\ a_1=\textit{first term's value}\\ r=\textit{common ratio}\\ ----------\\ a_1=10\\ n=5\\ r=\frac{1}{5} \end{cases}$

$\bf S_5=10\left( \cfrac{1-\left( \frac{1}{5} \right)^5}{1-\frac{1}{5}} \right)\implies S_5=10\left( \cfrac{1-\frac{1}{3125}}{\frac{4}{5}} \right) \\\\\\ S_5=10\left( \cfrac{\frac{3124}{3125}}{\frac{4}{5}} \right)\implies S_5=10\cdot \cfrac{781}{625}\implies S_5=\cfrac{1562}{125}$