Four corners of a quadrilateral and 360 degrees AFD=90 degrees.

AED=90 degrees

If α= 90 degrees

EDF is also equal to 90 degrees and becomes a square.

Question 18 is similar:

Triangle AED and triangle BEF are congruent

Because angle DEA= angle EBF=90 degrees

AD is parallel to FC, so angle F= angle ADE.

And ABCD is square, so BC=BF=AD.

So AD=BF

(ASA) So DE=EF, let BC=X, so DC = X. In triangle BCD, according to Pythagorean theorem BD= root number 2X, because the diagonal of the square is divided vertically, BO=DO = root number 2X, because DE ratio DF=DO ratio DB= 1 EO=2, and angle FDB= angle EDO, so the triangle DED.