That's actually a lot less clearer unfortunately. There's a great deal of business over the diagram and notations rotated, etc. Much more difficult to read than the psuedo-geometrical proof you were presenting.

You'd previously showed each step individually, which was an extremely clear communication - although, as noted, it broke down at a certain point. Refer to my previous comments re that.



As to the angle & mathematics, refer here: [en.wikipedia.org]

basic trig functions will yield that angle's value for you.