\begin{equation*}
\begin{split}
& \frac{\cos 50 - \cos 40}{\sin 50 - \sin 40} \\\\
& \frac{-2 \sin \frac{1}{2} (50 + 40) \sin \frac{1}{2}(50 - 40)}{2 \cos \frac{1}{2}(50 + 40) \sin \frac{1}{2}(50 -40)} \\\\
& \frac{-2 \sin 45 \sin 5}{2 \cos 45 \sin 5} \\\\
& - \tan 45 \\\\
& -1
\end{split}
\end{equation*}
