SG. 1 Given $\tan \theta=\frac{5}{12}$, where $180^{\circ} \leq \theta \leq 270^{\circ}$. If $A=\cos \theta+\sin \theta$, find the value of $A$.
$\begin{array}{l}\cos \theta=-\frac{12}{13}, \sin \theta=-\frac{5}{13} \\ A=-\frac{12}{13}-\frac{5}{13}=-\frac{17}{13}\end{array}$