Please Help with the proof of Arithmetic and Geometric mean theorem?

This one website proves the theorem, but I don't understand one part.

Here is the website:

http://jwilson.coe.uga.edu/emt725/AMGM/AMGM.3.html


I was wondering why are you allowed to add "4ab" to each side? Where does the "4ab" come from?


Thanks in advance.
2 answers 2