\cot (x+y) = \frac{\cot x \cot y - 1}{\cot x + \cot y}
$i.e., prove $ \tan (x+y) = \frac{\cot x + \cot y}{\cot x \cot y - 1} \ ($Reciprocating both sides$)
$LHS$ = \tan (x+y) = \frac{\tan x + \tan y}{1 - \tan x \tan y} \ ($By the tan expansion$)
= \frac{\cot x + \cot y}{\cot x \cot y - 1}...