\[\boxed{\text{409\ (409).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[(x - 5)^{2} + (y - 7)^{2} = r^{2}\]
\[\textbf{а)}\ точка\ касания\ (5;0):\ \ \ \]
\[(5 - 5)^{2} + (0 - 7)^{2} = 49;\]
\[\Longrightarrow r^{2} = 49 \Longrightarrow r = 7.\]
\[\textbf{б)}\ точка\ касания\ (0;7):\ \ \ \]
\[(0 - 5)^{2} + (7 - 7)^{2} = 25;\]
\[\Longrightarrow r^{2} = 25 \Longrightarrow r = 5.\]