\[\boxed{\mathbf{343.}ОК\ ГДЗ\ –\ домашка\ на\ 5}\]
\[Дано:\]
\[Решение.\]
\[S_{осн} = \pi R^{2};\]
\[\frac{d}{\sqrt{2}} - сторона\ квадрата.\]
\[\frac{d}{\sqrt{2}} = 2\pi R;\]
\[R = \frac{d}{2\sqrt{2} \cdot \pi}.\]
\[S_{осн} = \pi \cdot \frac{d^{2}}{8 \cdot \pi^{2}} = \frac{d^{2}}{8\pi}.\]
\[Ответ:\frac{d^{2}}{8\pi}.\]