$(a)\;(0, {\large\frac{\pi}{4}}] \qquad(b)\;[\large\frac{\pi}{6}, {\large\frac{\pi}{3}}] \qquad(c)\;[\large\frac{\pi}{4}, {\large\frac{\pi}{3}}]\qquad(d)\;(\large\frac{\pi}{3}, {\large\frac{\pi}{2}}]$