* Bạn tham khảo nha >v< *
$\begin{align} & \,\tan {{9}^{o}}-\tan {{63}^{o}}+\tan {{81}^{o}}-\tan {{27}^{o}} \\ & =\tan {{9}^{o}}+\cot {{9}^{o}}-\left( \tan {{27}^{o}}+\cot {{27}^{o}} \right) \\ & =\dfrac{\sin {{9}^{o}}}{\cos {{9}^{o}}}+\dfrac{\cos {{9}^{o}}}{\sin {{9}^{o}}}-\left( \dfrac{\sin {{27}^{o}}}{\cos {{27}^{o}}}+\dfrac{\cos {{27}^{o}}}{\sin {{27}^{o}}} \right) \\ & =\dfrac{1}{\sin {{9}^{o}}\cos {{9}^{o}}}-\dfrac{1}{\sin {{27}^{o}}\cos {{27}^{o}}} \\ & =\dfrac{2}{\sin {{18}^{o}}}-\dfrac{2}{\sin {{54}^{o}}} \\ & =\dfrac{4\cos {{36}^{o}}\sin {{18}^{o}}}{\sin {{18}^{o}}\sin {{54}^{o}}} \\ & =4 \\ \end{align}$
