In the next formula, each term is half as large as the previous term.
Even when we don't know the exact number of terms, we can still say:
It may be somewhat surprising that the sum is less than 1 no matter how many terms there are. Figure C-3 shows why this is true.
Figure C-3. When the terms of a sum of halves are rearranged, they don't quite fill up a 2 x 1/2 rectangle. The missing piece is precisely the size of the last term: 1/2n.
It is sometimes more convenient to write a sum in which each term is twice (rather than half) the previous term.
C 5 Upper Limit on Sum of a Function