# Simplification of expected reward under the limit in continuous tasks

I was reading the average reward setting for continuous tasks from rich sutton's book (page 202, 2nd edition). There he perform a simplification over the expected reward under the limit approaching to infinite. I mark this point in this picture:

The book does not clearly mention the steps to simplify the above expression. I search on the web to find the solution but there is no clear explanation on that. Can anyone explain the marked point?