∵ABCD是平行四边形,又∠ABC＝90°,∴平行四边形ABCD是矩形,
∴∠BAD＝∠ECF＝90°、AB＝DC.
∵∠BAD＝90°、∠BAE＝∠BAD/2,∴∠BAE＝45°,又∠ABE＝90°,∴AB＝BE.
由AB＝DC、AB＝BE,得：DC＝BE.
∵∠BAE＝45°、AB＝BE,∴∠AEB＝45°,∴∠CEF＝45°,又∠ECF＝90°,∴CE＝CF.
由DC＝BE、CF＝CE,得：DC＋CF＝BE＋CE,∴DF＝BC.
∵∠ECF＝90°、EG＝FG,∴CG＝FG、∠DFG＝∠BCG＝45°.
由DF＝BE、FG＝CG、∠DFG＝∠BCG,得：△DFG≌△BCG,∴∠CDG＝∠CBG,
∴B、G、C、D共圆,∴∠BDG＝∠BCG＝45°