แนวคิด ข้อ 4
$\begin{array}{l}
\cot \left( {A + B} \right) = \cot \left( {\pi - C} \right)\\
\dfrac{{\cot A\cot B - 1}}{{\cot A + \cot B}} = - \cot C\\
\cot A\cot B - 1 = - \cot C\cot A - \cot C\cot B\\
\cot A\cot B + \cot B\cot C + \cot C\cot A = 1
\end{array}$
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