\[1)\ f(x) = \sin x + \cos x;x_{0} = \frac{\pi}{2}:\]
\[f^{'}(x) = \left( \sin x \right)^{'} + \left( \cos x \right)^{'} =\]
\[= \cos x - \sin x;\]
\[f^{'}\left( \frac{\pi}{2} \right) = \cos\frac{\pi}{2} - \sin\frac{\pi}{2} =\]
\[= 0 - 1 = - 1.\]
\[Ответ:\ \ k = - 1.\]
\[2)\ f(x) = \cos{3x};\ \ x_{0} = \frac{\pi}{6}:\]
\[f^{'}(x) = \left( \cos{3x} \right)^{'} = - 3\sin{3x};\]
\[f^{'}\left( \frac{\pi}{6} \right) = - 3\sin\frac{3\pi}{6} =\]
\[= - 3\sin\frac{\pi}{2} = - 3.\]
\[Ответ:\ \ k = - 3.\]