Cho \(\sin a = \frac{2}{3}\) với \(\frac{\pi }{2} < a < \pi \). Tính: \(\sin \left( {a + \frac{\pi }{4}} \right),\,\cos \left( {a - \frac{{5\pi }}{6}} \right),\,\tan \left( {a + \frac{{2\pi }}{3}} \right)\);