(Chứng minh (*):
$\tan(\frac{A}{2}+\frac{B}{2})=\frac{\tan\frac{A}{2}+\tan\frac{B}{2}}{1-\tan\frac{A}{2}.\tan\frac{B}{2}}\Leftrightarrow\frac{1}{\tan\frac{C}{2}}=\frac{\tan\frac{A}{2}+\tan\frac{B}{2}}{1-\tan\frac{A}{2}.\tan\frac{B}{2}}$
$\Rightarrow (*)$
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