Question:

I attempted to answer it.

The problem I have is with the worked solutions is how did the author come up with:

I thought that

therefore r = common ratio = r^{-1}

and lastly how did the author need up with where as the answer

I thought is wrong.

Shouldn't ?

Please if someone can be kind to explain to me thank you

Edit: Please don't worry about the second part of the question

I did this question