G1.3 Given that $\tan \alpha=-\frac{1}{2}$. If $c=\frac{2 \cos \alpha-\sin \alpha}{\sin \alpha+\cos \alpha}$, find the value of $c$.
$\tan \alpha=-\frac{1}{2} \cdot c=\frac{2 \cos \alpha-\sin \alpha}{\sin \alpha+\cos \alpha}=\frac{2-\tan \alpha}{\tan \alpha+1}=\frac{2+\frac{1}{2}}{-\frac{1}{2}+1}=5$