Найдите сумму бесконечной геометрической прогрессии 28; -14; 7; …

\[b_{1} = 28;\ \ \ b_{2} = - 14;\]

\[q = \frac{b_{2}}{b_{1}} = - \frac{14}{28} = - \frac{1}{2} < 1;\]

\[S = \frac{b_{1}}{1 - q} = \frac{28}{1 + \frac{1}{2}} = \frac{28}{\frac{3}{2}} =\]

\[= 28 \cdot \frac{2}{3} = \frac{56}{3} = 18\frac{2}{3}.\]