not sure yet but looks like the denominator 1 - (0.8/0.5) may be taken care of using forumla (a - b)(a + b) = a^2 - b^2 ....
In one of my textbook example, after the summation result was given, they simplified the expression but I can't see how this simplification was done.
I don't see how the LHS simplified to the RHS? All help will be appreciated. Thanks in advance.