[m]b_{n}=b_{1}\cdot q^{n-1}[/m]
n=6
b_(1)= –64; q=1/2
[m]b_{6}=(-64)\cdot (\frac{1}{2})^{6-1}[/m]
[m]b_{6}=(-64)\cdot (\frac{1}{2})^{5}[/m]
[m]b_{6}=(-64)\cdot \frac{1}{32}[/m]
[m]b_{6}=-2[/m]
[m]S_{n}=\frac{b_{1}\cdot (1-q^{n})}{1-q}[/m]
n=5
[m]S_{5}=\frac{(-64)\cdot(1-(\frac{1}{2})^{5})}{1-\frac{1}{2}}[/m]
[m]S_{5}=\frac{(-64)\cdot(1-\frac{1}{32})}{\frac{1}{2}}[/m]
[m]S_{5}=\frac{(-64)\cdot\frac{31}{32})}{\frac{1}{2}}[/m]
[m]S_{5}=-124[/m]