If A + B + C = π, show that...? [Trig]

If A + B + C = π, show that (tanA + tanB)/(1- tanAtanB) = -tanC.
(using addition identities for tan(x ± y))

Please show full working.

Thanks.

A + B + C = Pi
A + B = Pi - C
tan (A + B) = tan (pi - C)
(tan A + tan B)/(1 - tan A.tan B) = - tan C (since Pi - C in the IInd Quarter)

(tan(A) + tan(B))/(1- tan(A)tan(B))
= tan(A + B)
A+B = π - C
tan(π - C) = -tan(C)