问题
下面【三角函数 $\cot$ 的二倍角公式】中，正确的是哪个？选项
[A]. $\cot 2 \alpha =$ $\frac{2 \cot \alpha}{\cot ^{2} \alpha – 1}$[B]. $\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha – 2}{2 \cot \alpha}$
[C]. $\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha + 1}{2 \cot \alpha}$
[D]. $\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha – 1}{2 \cot \alpha}$
$\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha – 1}{2 \cot \alpha}$