# 3D trigonometry solving problem

For the following question, I wonder if it can be solved without using any double angle trig identities as well as any sum and difference trig formulas. The reason is because I haven't learnt those formulas but I tried to look them up and apply into my working out. However, I'm afraid my way may not be "valid" since i haven't learnt the double angle formula for tan at school. I tried to prove the formula I used but did not get to anywhere since apparently if I want to prove it, I need to apply double angle formulas for cos and sine. I've attached the question and my working out below and hope someone would be able to have a look at them. Thank you so much for helping! (question 12)

• Welcome to Math.SE. Please write the question and your answer in here, using MathJax so that it's easier for somebody to help you.. – Ertxiem Apr 1 at 21:37