回復 12# royan0837 的帖子
填充第 4 題
\(\begin{align}
& \cot \left( \theta +\phi \right)=1 \\
& \frac{\cot \theta \cot \phi -1}{\cot \theta +\cot \phi }=1 \\
& \left( \cot \theta -1 \right)\left( \cot \phi -1 \right)=2 \\
& \cot \theta =3,\cot \phi =2 \\
\end{align}\)
