### Home > PC3 > Chapter 12 > Lesson 12.1.2 > Problem12-26

12-26.

Use Austin’s method to compute the sum of each series below.

1. $1 + 5 + 25 + 125 + 625 + 3125 + 15,625$

If the given series is $S$, compute $5S − S$.

Did you get $4S = 78{,}124 - 1$?

2. $5 + 15 + 45 + . . . + 885,735$

Identify the common ratio, $r$.
Then, if the given series is $S$, compute $rS − S$.