Question

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

Answer 1

if you want to find the second term multiply the first term \(\large\color{blue}{a_1 }\)
times the common ratio \(\large\color{blue}{r }\).

In other words, \(\large\color{blue}{10 \times \frac{1}{5} }\) . This is the second term.

Answer 2

then multiply the second term that you get times 1/5 to get the third one.
then the third term times 1/5 to get the 4th one.

... you get the pattern, right?

Answer 3

then add the terms to find the sum.
OR, do you want it just written in a sigma notation?