As shown in the figure, the overlapping area of two squares =S△BOE+S△BOE.

Where: ∠ BOE+∠ AOE = ∠ AOB = 90.

∠BOF+∠BOE=∠FOE=90

∴∠BOF=∠AOE

≈OAE =∠OBF = 45° OA = OB。

∴△BOF≌△AOE

I won't write it later. Sometimes the difference between computer and handwriting is so big that I'm really not used to it.