$(a)\;\large\frac{\pi}{2} - \large\frac{2}{3}\qquad(b)\;\large\frac{\pi}{2} + \large\frac{2}{3}\qquad(c)\;\large\frac{\pi}{2} + \large\frac{4}{3}\qquad(d)\;\large\frac{\pi}{2} - \large\frac{4}{3}$