Analysis: Let the side length of a square be X, and work out the side lengths of two squares and three squares according to the properties of squares, Pythagorean theorem and similar triangles's judgment and properties, from which the side length laws of n squares can be obtained, so as to get the answer to the question.

Answer:

Solution:

Such as 1 square, as shown in the figure.

Point A is AM⊥BC, vertical foot is M, and point N is GH.

∴∠AMC=90，

∵ quadrilateral EFGH is a square,

∴GH∥BC,GH=GF,GF⊥BC，

∴∠AGH=∠B,∠ANH=∠AMC=90。

∠∠GAH =∠BAC，

∴△AGH∽△ABC.

∴AN:AM=GH:BC，

The area of ABC is 12, and BC is 6.

∴ s △ ABC =1/2 (BC× am) =1/2× 6× am =12, and the solution is AM = 4.

Let GH=x, BC=6, AM=4,

GF = NM = GH，

∴AN=AM-NM=AM-GH=4-x，

∴x/6=(4-x)/4,x=/ 12/5，

Similarly, when n=2, x= 12/7,

Therefore, for n squares, x= 12/(2n+3),

So choose: D.