DC = ed

In rectangular ABCD

AB=DC

Then AB=ED

Two right-angled triangles ABF, EDF

Diagonal ∠AFB=∠EFD

You can prove that two right-angled triangles are consistent.

That is △ ABF △ EDF.

Then FB=FD.

(2) The aforementioned evidence FB=FD

Delta delta △FBD

∠FDB=∠FBD

Outside angle ∠AFB=∠FDB+∠FBD=2∠FDB

In the right triangle AFB

ABF=50

Then < AFB = 90-50 = 40.

That is 2 ∠ FDB = 40.

So ∠ FDB = 20.