I think we're missing some information. I believe the identity is true only if A, B, and C are angles of the same triangle (A + B + C = 180°).
If so, break up the tangents into sines and cosines.
Then try using the product to sum formulas:
Ok I have checked the information and I left out the first line and you were correct, it only applies to a triangle.
On the level that we do trigenometry, we can do compound angles
Then changing from
And from
Do you perhaps know how i would go about proving this identity using these rules?