По теореме синусов:
\(\frac{AB}{sin C} = 2R\), где R - радиус описанной окружности.
Дано: \(AB = 26\sqrt{2}\), \(C = 135^\circ\)
\(sin(135^\circ) = sin(180^\circ - 45^\circ) = sin(45^\circ) = \frac{\sqrt{2}}{2}\)
\(2R = \frac{26\sqrt{2}}{\frac{\sqrt{2}}{2}} = 26\sqrt{2} * \frac{2}{\sqrt{2}} = 26 * 2 = 52\)
\(R = \frac{52}{2} = 26\)
Ответ: 26