แนวคิด ข้อ 3
$\begin{array}{l}
\tan \left( {A + B} \right) = \tan \left( {\pi - C} \right)\\
\dfrac{{\tan A + \tan B}}{{1 - \tan A\tan B}} = - \tan C\\
\tan A + \tan B = - \tan C + \tan A\tan B\tan C\\
\tan A + \tan B + \tan C = \tan A\tan B\tan C
\end{array}$
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