tan3xtan2xtanx = tan3x - tan2x - tanx

3x = 2x + x so, tan(3x) = Tan (2x + x)

tan3x = (tan 2x + tanx ) / (1- tanx*tan2x)  using identity tan (A + B) = tan A + tanB / 1-tanA* tanB

cross multiply

tan3x - tan3xtan2x tanx = tanx + tan2x

= tan3x - tan2x - tanx = tan3xtan2xtanx