3. If $\frac{\cot \theta-1}{2 \cot \theta+1}=1$, then the value of $\cos 2 \theta$ is
A. $\frac{3}{5}$
B. $-\frac{3}{5}$
C. $\frac{2 \sqrt{5}}{5}$
D. $-\frac{2 \sqrt{5}}{5}$
3. A Solving, we get $\cot \theta=-2$ so $\tan \theta=-\frac{1}{2}, \cos 2 \theta=\frac{1-\tan ^{2} \theta}{1+\tan ^{2} \theta}=\frac{3}{5}$