https://astarmathsandphysics.com/ib-maths-notes/trigonometry/4967-when-is-the-identity-tanatanbtanc-tana-tanb-tanc-valid.html
When is the Identity tanAtanBtanC=tanA+tanB+tanC Valid?
IB Maths Notes - Trigonometry - When is the Identity tanAtanBtanC=tanA+tanB+tanC Valid?
identitytanatanbvalid