(7) The slope of the tangent line at a point on the graph of the function $f(x)=x+\cos x-\sqrt{3} \sin x$ is $k$, then the range of values for $k$ is $\qquad$ .
(7) $[-1,3]$ Hint:
$$
f^{\prime}(x)=1-\sin x-\sqrt{3} \cos x=1-2 \sin \left(x+\frac{\pi}{3}\right) \in[-1,3] .
$$