\[\boxed{\mathbf{1501}\mathbf{.}}\]
\[y = \frac{2}{3}\cos\left( 3x - \frac{\pi}{6} \right);\]
\[касательная\ в\ точке\ x_{0} = \frac{\pi}{3}:\]
\[y^{'}(x) = \frac{2}{3}\left( \cos\left( 3x - \frac{\pi}{6} \right) \right)^{'} =\]
\[= \frac{2}{3} \bullet ( - 3) \bullet \sin\left( 3x - \frac{\pi}{6} \right) =\]
\[= - 2\sin\left( 3x - \frac{\pi}{6} \right);\]
\[y^{'}\left( \frac{\pi}{3} \right) = - 2\sin\left( \frac{3\pi}{3} - \frac{\pi}{6} \right) =\]
\[= - 2\sin\left( \pi - \frac{\pi}{6} \right) = - 2\sin\frac{\pi}{6} =\]
\[= - 2 \bullet \frac{1}{2} = - 1.\]
\[Угол\ между\ касательной\ и\ \]
\[осью\ Ox:\]
\[a = arctg\ k = arctg\ ( - 1) = - \frac{\pi}{4}.\]
\[Ответ:\ \ a = - \frac{\pi}{4}\text{.\ }\]