\[\boxed{\mathbf{965.ОК\ ГДЗ - домашка\ на}\ 5}\]
\[\mathbf{Уравнение\ окружности:}\]
\[\left( x - x_{0} \right)^{2} + \left( y - y_{0} \right)^{2} = r^{2};\]
\[x_{0} = y_{0} = 0.\]
\[\textbf{а)}\ r = 3:\ \ \ \]
\[\ x^{2} + y^{2} = 9.\]
\[\textbf{б)}\ r = \sqrt{2}:\ \ \ \ \]
\[x^{2} + y^{2} = 2.\]
\[\textbf{в)}\ r = \frac{5}{2}:\ \ \ \ \]
\[x^{2} + y^{2} = \frac{25}{4}.\]