$\displaystyle cot(A+B) = \frac{cotAcotB-1}{cotA+cotB} $

so far I have done;

$\displaystyle \frac{ \frac{1}{tanA} \times \frac{1}{tanB} - 1}{ \frac{1}{tanA} + \frac{1}{tanB}} $

which after simplifying I get; $\displaystyle \frac{1-tanAtanB}{tanA + tanB} $

I am stuck here, if anyone could direct me from here, thanks.