In this recent question, the equation $$\tan\left(\frac{1}{A}\right) = \tan\left(\frac{1}{B}\right) + \tan\left(\frac{1}{C}\right)$$ is said to imply $$A + B + C = ABC$$ without any stated constraints. Where does this come from, and is it even correct?
I must be misunderstanding the context here, because even with integers like $B = 4$ and $C = 5$, the equations disagree.