\[\boxed{\mathbf{772}\mathbf{.}}\]
\[y = tg\ 2x - 1\]
\[tg\ 2x - 1 < 0\]
\[tg\ 2x < 1\]
\[- \frac{\pi}{2} + \pi n < 2x < arctg\ 1 + \pi n\]
\[- \frac{\pi}{2} + \pi n < 2x < \frac{\pi}{4} + \pi n.\]
\[Ответ:\ - \frac{\pi}{4} + \frac{\text{πn}}{2} < x < \frac{\pi}{8} + \frac{\text{πn}}{2}.\]