\[\boxed{\mathbf{974.}}\]
\[1)\ \frac{\pi}{2} - a\]
\[0 < a < \frac{\pi}{2}\]
\[- \frac{\pi}{2} < - a < 0\]
\[0 < \frac{\pi}{2} - a < \frac{\pi}{2}\]
\[в\ I\ четверти.\]
\[2)\ a - \pi\]
\[0 < a < \frac{\pi}{2}\]
\[- \pi < a - \pi < - \frac{\pi}{2}\ \ \ \ \ | + 2\pi\]
\[\pi < a - \pi < \frac{3\pi}{2}\]
\[в\ III\ четверти.\]
\[3)\ \frac{\pi}{2} + a\]
\[0 < a < \frac{\pi}{2}\]
\[\frac{\pi}{2} < \frac{\pi}{2} + a < \pi\]
\[во\text{\ II\ }четверти.\]
\[4)\ \pi - a\]
\[0 < a < \frac{\pi}{2}\]
\[- \frac{\pi}{2} < - a < 0\]
\[\frac{\pi}{2} < \pi - a < \pi\]
\[во\text{\ II\ }четверти.\]