Presumably you know that the sum of a geometric series . With a= 1, r= q, that is .
Taking the reciprocal of both sides gives
the next to the last formula. Notice that there are terms in that sum. As the book says, as n goes to infinity, all those powers of q go to 1 so we are adding 1 times- the sum is so the left side is .
Putting that into the fourth formula on the page,
gives the final result.