-21/64
1/64
-20/64
1/16
b_(2)+b_(4)=5/16
b_(1)+b_(1)*q^2=-5/8
b_(1)*q+b_(1)*q^3=5/16
Решаем систему двух уравнений с двумя неизвестными
{b_(1)*(1+q^2)=-5/8
{b_(1)*q*(1+q^2)=5/16
{[b]b_(1)*(1+q^2)[/b]=-5/8
{[b]b_(1)*[/b]q*[b](1+q^2)[/b]=5/16 ⇒ (-5/8)*q=5/16
[m]q=\frac{5}{16}:(-\frac{5}{8})[/m]
[m]q=-\frac{1}{2}[/m]
b_(1)*(1+q^2)=-5/8
b_(1)*(1+[m](\frac{1}{2})^2[/m])=-5/8 ⇒ b_(1)=-1/2
[m]S_{6}=\frac{b_{1}(1-q^{6})}{1-q}[/m]
[m]S_{6}=\frac{(-\frac{1}{2})\cdot (1-(-\frac{1}{2})^6)}{1-(-\frac{1}{2})}[/m]
[m]S_{6}=\frac{(-\frac{1}{2})\cdot(1-\frac{1}{64})}{\frac{3}{2}}[/m]
[m]S_{6}=-\frac{21}{64}[/m]