Chapter 7 - Section 7.3 - Geometric Sequences and Series - 7.3 Exercises - Page 654: 25

$S_5=860.9513637615\approx 860.95$

In a geometric series the ratio between consecutive terms is constant. The $S_n$, so the sum of the first n terms can be written as: $S_n=a_1\times \frac{1-r^{n}}{1-r}$ Here, $n=5$, $a_1=8.423$, $r=2.859$: $S_n=a_1\times \frac{1-r^{n}}{1-r}$ $S_5=8.423\times \frac{1-(2.859^{5})}{1-(2.859)}=860.9513637615\approx 860.95$