{y_(1)+y_(1)*q^2=-10
{y_(1)*q+y_(1)*q^3=-20
{[b]y_(1)*(1+q^2)[/b]=-10
{y_(1)*q*(1+q^2)=-20
q*[b]y_(1)(1+q^2)[/b]=-20
q*(-10)=-20
q=2
y_(1)*(1+2^2)=-10
y_(1)=-2
S_(7)=[m]\frac{y_{1}\cdot(q^7-1)}{q-1}=\frac{y_{1}\cdot(q^7-1}{q-1}=-2\cdot 127=-254[/m]