I'm guessing the sum is supposed to be
Split the summand into partial fractions:
If 
, then
If 
, then
This means
Consider the 
th partial sum of the series:
The sum telescopes so that
and as 
, the second term vanishes and leaves us with
Here is an example:
Josh bought 7 apples for $21.
21/7 is 3, so one apple costs $3. This is the unit rate.
21/7 simplifies to 3/1, that’s another way to think about it. Simplify the fraction until the denominator is 1.
Hope this helps! Tell me if you have any questions!
