Problem
9 Partial sums of geometric series 9Ju Find the fourth partial sum of the series. \[ 5+25+125+625+3125+15625+\cdots \] Write your answer as an integer or a fraction in simplest form \[ S_{4}= \]
Solution
Step 1 ：\(a_1 = 5\)
Step 2 ：\(r = 5\)
Step 3 ：\(S_{4} = \frac{a_1(r^n - 1)}{r - 1} = \frac{5 (5^4 - 1)}{5 - 1}\)
