This formula is valid if, . To see this you need to know that
(1) , by definition of
and (2) that .
Thus, overall you get that
what was to be shown.
If you know that , it is easy to see that if , it follows that .and
are given formulas? (if anyone knows?)
and finally I do not understand how to get from the step where I worked out all z's in my brackets to an answer that makes sense.
Where the equivalence that I have marked red follows from the above formula , if you subtitute for and for .