regarding the second exercise:
equals one for any integral n:
In the first picture, there are two circles i've indicated. My question is how to get the second one from the first circle. I can see that n changed from n≠ 0 to n=1. And I also don't understand why I have to change these n values.
Moreover, in the second picture, I wasn't sure why that circled one has disappeared in the next equation.
Thanks in advance!