\[\begin{array}{l} {\text{If }}\alpha {\text{, }}\beta ,{\text{ }}\gamma {\text{ are angles of a triangle then}} \hfill \\ \operatorname{tg} \alpha + \operatorname{tg} \beta + \operatorname{tg} \gamma = \operatorname{tg} \alpha \cdot \operatorname{tg} \beta \cdot \operatorname{tg} \gamma . \hfill \\ \end{array} \]