Đáp án:
$\begin{array}{l}
c{\rm{os}}\,{\rm{y + siny}}{\rm{.tany }}\\
{\rm{ = }}\,c{\rm{os}}\,{\rm{y + siny}}{\rm{.}}\frac{{{\mathop{\rm s}\nolimits} {\rm{inx}}}}{{{\rm{cosx}}}}\\
= \frac{{{{\cos }^2}x + {{\sin }^2}x}}{{\cos x}}\\
= \frac{1}{{{\rm{cos x}}}}
\end{array}$