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Mathematical phoenix edition
Solution: ABCD is an isosceles ladder, which is easy to know.

OB=OC,OA=OD

∫∠ADB = 60

∴△ADO and △OBC are equilateral triangles.

AE = EO,OF=BF

∴EF=AB/2,DE⊥AC,CF⊥BD

DG = GC

∴EG=DG=GC=DC/2=AB/2

FG==DG=GC=DC/2=AB/2

∴EF=EG=FG

Triangle EFG is an equilateral triangle.