(1)
∵DEFG为正方形
∴DE//GF
∴DM/BG=AM/AG
MN/GF=AM/AG
∴DM/BG=MN/GF
(2)
∵∠BAC=90°,
∴∠C=∠GDB,∠B=∠CEF
∴CF/EF=DG/BG,又EF=DG=GF
∴CF*GB=GF*GF
又DM/BG=MN/GF=EN/CF
∴DM*EN/(BG*CF)=MN*MN/(GF*GF)
即:DM*EN=MN*MN
(3)
AB=AC=2
则CF=EF=GF=GD=GB
∴EF=CF=BC/3
MN/GF=(BC/2-BC/3)/(BC/2)
MN=GF/3=BC/9=0.157