∠∠AGB = 90°, ∠ Gabe = 30.

∴AB=2GB=4,∠ABG=90-∠GAB=60

Square ABCD

∴CD=AB=4，∠ABC=90

∴∠CBG=∠ABC-∠ABG=90-60=30

∵GH⊥BC

∴gh=gb/2= 1,bh=gb×√3/2=2×√3/2=√3

∴CH=BC-BH=4-√3

∴CG？ =GH？ +CH？ = 1+(4-√3)? =20-8√3

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