Решебник по геометрии 9 класс Мерзляк Задание 244

\[R = \frac{a}{2\sin\frac{180{^\circ}}{4}} = \frac{a}{2\sin{45{^\circ}}} =\]

\[= \frac{a}{2}\ :\frac{\sqrt{2}}{2} = \frac{a}{\sqrt{2}}.\]

\[S = \pi R^{2} = \pi \bullet \left( \frac{a}{\sqrt{2}} \right)^{2} =\]

\[= \pi \bullet \frac{a^{2}}{2} = \frac{\pi a^{2}}{2}.\]

\[Ответ:\ \ \frac{\pi a^{2}}{2}.\]