Packing 6 circles in a Square

For a maths task I'm trying to prove that this http://www2.stetson.edu/~efriedma/cirinsqu/cs6.gif is the most efficient way of packing 6 circles into a square on a plane.

I've read the wiki article http://en.wikipedia.org/wiki/Circle_packing_in_a_square but it doesn't make sense to me. I can't understand how 'Dn' was calculated for example. Any hints to get me on the right track?

Any and all help would be greatly appreciated :)

Try visiting http://hydra.nat.uni-magdeburg.de/packin…