Solution: Let CH intersect with DE and BG at M point and N point respectively, the side length of the square is 3a, DH=CG=a,

Firstly, it is concluded that the shadow part is square and △ ade △ dch from the central symmetry of the square, and it is proved that DM⊥CH,

In Rt△CDH, CH= 10a is obtained from Pythagorean theorem, and 12CH is obtained from area formula. DM= 12DH？ laser record

Get DM=3 10 10a,

MH= 10 10a is obtained from Pythagorean theorem in Rt△DMH.

Then Mn = ch-MH-CN =10A-310A-1010A = 3510A,

So shadow area: square area ABCD = 9025a2: 9a2 = 2: 5.

So choose a.