1. The problem statement, all variables and given/known data I'm doing quite a strict proof in school. Where we should proof something and use mathematical language and symbols. 2. Relevant equations 3. The attempt at a solution To proof what I have to proof I need to draw some help lines. As for instance the "red" one I did from A to B. However I'm very unsure how to write this in mathematical symbols that I draw a helpline to form a triangle ABC (If you imagine that the intersection between A and B is called C. Any tips on how to write this in a neet mathematical way?