If the co-domain of the function
\[
f(x)=
\begin{cases}
3\sin x-4\cos x, & x\in \left(\tan^{-1}\frac{4}{3}-\frac{\pi}{2},\,\tan^{-1}\frac{4}{3}+\frac{\pi}{2}\right)\\[2mm]
\log(\sin x), & x\in \left(\frac{5\pi}{6},\pi\right)
\end{cases}
\]
is \((-\infty,5)\), then \(f\) is