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- Hướng dẫn giải

Ta có: $\alpha +\beta +\gamma =\pi \Rightarrow \alpha +\beta =\pi -\gamma$

$\Rightarrow 3\left(\alpha +\beta \right)=3\pi -3\gamma =2\pi +\pi -3\gamma$.

 Do đó,

$$\begin{gathered} \cos \left(3\alpha +3\beta \right)=\cos \left(2\pi +\pi -3\gamma \right)=\cos \left(\pi -3\gamma \right)=-\cos \left(-3\gamma \right)=-\cos 3\gamma \\ \sin \left(3\alpha +3\beta \right)=\sin \left(2\pi +\pi -3\gamma \right)=\sin \left(\pi -3\gamma \right)=-\sin \left(-3\gamma \right)=\sin 3\gamma \\ \tan \left(3\alpha +3\beta \right)=\frac{\sin \left(3\alpha +3\beta \right)}{\cos \left(3\alpha +3\beta \right)}=\frac{\sin 3\gamma }{-\cos 3\gamma }=-\tan 3\gamma \\ cot\left(3\alpha +3\beta \right)=-cot3\gamma \end{gathered}$$