2).
[tex] b_{8} = b_{1}* q^{8-1} \\ b_{8}= 3 * (-1) ^{7} \\ b_{8}= 3*(-1)= -3[/tex]
4).
[tex]S_{10} = b_{1} * \frac{q^{10}-1}{q-1}=4 * \frac{ \frac{1}{2^{10}} -1 }{- \frac{1}{2} } = \\ \\
= -2^{2} * \frac{ \frac{1-2^{10}}{2^{10}} }{ \frac{1}{2} } =-2^2* \frac{1-2^{10}}{2^9}= \\ \\
= - \frac{1-2^{10}}{2^7}= - \frac{1-2^{10}}{128} = - \frac{1-1024}{128}= - \frac{-1023}{128} = \\ \\ = \frac{1023}{128} [/tex]
