Q: If A + B + C = π, prove tan A + tan B + tan C = tan A tan B tan C. tan(A+B) = tan(π-C) = -tan C. (tanA+tanB)/(1-tanAtanB) = -tanC. tanA + tanB = -tanC + tanAtanBtanC. Rearrange: tanA+tanB+tanC = tanAtanBtanC. ✓