Just one question
How did he find this angle?
Need explain
Dear TWiX,
The area bounded by $\displaystyle F_{1}$ , $\displaystyle F_{2}$ and the two dotted lines must be a parallogram.
Then, $\displaystyle F_{1}\hat{O}F_{2}=60^{o}+45^{o}=105^{o}$
Therefore, $\displaystyle \psi=180^{o}-105^{o}=75^{0}$
Hope this helps.