[tex]\tan 195^{\circ}\\\\=\tan\left(3 \times 90^{\circ} - 75^{\circ} \right)\\\\=\cot 75^{\circ}\\\\=\left(\tan 75^{\circ} \right)^{-1}\\\\= \textbf{[}\tan \left(30^{\circ} + 45^{\circ}\right) \right \textbf{]}^{-1}\\\\=\left(\dfrac{\tan 30^{\circ} +\tan 45^{\circ}}{1- \tan 45^{\circ} \cdot \tan 30^{\circ}} \right)^{-1}\\\\\\=\left( \dfrac{\dfrac 1{\sqrt 3}+1}{1- \dfrac 1{\sqrt 3}} \right)^{-1}\\\\\\=\left( \dfrac{\sqrt 3+1}{\sqrt 3-1}\right)^{-1}\\\\\\=\dfrac{\sqrt 3-1}{\sqrt 3 +1}\\[/tex]
[tex]=\dfrac{\left( \sqrt 3-1\right)^2}{\left( \sqrt 3+1\right)\left( \sqrt 3-1\right)}\\\\\\=\dfrac{3+1-2\sqrt 3}{3-1}\\\\\\=\dfrac{4-2\sqrt 3}{2}\\\\\\=2-\sqrt 3[/tex]
