Chapter 9 - Section 9.3 - Geometric Sequences; Geometric Series - 9.3 Assess Your Understanding - Page 665: 79

$\approx 3.753\times 10^{14}$

$\displaystyle \frac{8}{-4}=\frac{-4}{2}=\frac{2}{-1}=-2$, a costant ratio. The sequence is geometric, $a_{1}=-1,r=-2$ The sum $S_{n}$ of the first $n$ terms is $S_{n}=\displaystyle \sum_{k=1}^{n}a_{1}r^{k-1}=a_{1}\cdot\frac{1-r^{n}}{1-r}$ $S_{50}=-1\displaystyle \cdot\frac{1-(-2)^{50}}{1-(-2)}=\frac{2^{50}-1}{3}\approx 3.753\times 10^{14}$