\[\boxed{\mathbf{901\ (901)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\angle AOB = 45{^\circ}\]
\[\angle AOC = 90{^\circ}\]
\[\angle AOD = 135{^\circ}\]
\[\angle DOB = \angle AOD - \angle AOB = 135 - 45 = 90{^\circ}.\]
\[Ответ:\ \ \angle DOB = 90{^\circ}.\]