Look at diagrams bellow :
We appended the image of himself then skewed it a bit.
The side of each triangle is the square root of the area of each surface !
We get as answer:
[display]\sqrt{S_{yellow}}+\sqrt{S_{blue}}+\sqrt{S_{green}}=\sqrt{S_{total}}[/display]
Yep, this is a rectangle and I use square root to find the side !
But's because rectangle and square differ only by length.