Thank you, now almost everything is clear for me except one thing. How do you know that:
I am sure you, as an expert, presented me it correctly, but i am just curious where it comes from. How do you know these identities? (they are not the common ones i think...)
You already have that .
But you also have that (it's a standard result, but simple to prove).
Multiply this out, you get
But as (which you should be able to prove, it's very straightforward), this works out as
The result follows from .