Найдите сумму бесконечной геометрической прогрессии 27, –9, 3, …

\[27;\ - 9;3;\ldots\]

\[b_{1} = 27;\ \ \ q = - \frac{9}{27} = - \frac{1}{3} < 1.\]

\[S = \frac{b_{1}}{1 - q} = \frac{27}{1 + \frac{1}{3}} = 27\ :1\frac{1}{3} = 27\ :\frac{4}{3} =\]

\[= 27 \cdot \frac{3}{4} = \frac{81}{4} = 20\frac{1}{4}.\]

\[Ответ:20\frac{1}{4}.\]