# Math Help - How in earth they find this angle??!

1. ## How in earth they find this angle??!

Just one question
How did he find this angle?
Need explain

2. Dear TWiX,

The area bounded by $F_{1}$ , $F_{2}$ and the two dotted lines must be a parallogram.

Then, $F_{1}\hat{O}F_{2}=60^{o}+45^{o}=105^{o}$

Therefore, $\psi=180^{o}-105^{o}=75^{0}$

Hope this helps.