(2) Established

Proof: from the topic, CE=DF, CD=DA, angle ECD= angle FDA,

So the triangle ECD is equal to the triangle FDA.

AF=DE, angle FAD= angle EDC.

Angle ADC=90 degrees, then angle FAD+ angle AFD=90 degrees.

So angle AFD+ angle EDC=90 degrees.

Get AF vertical DE

(3) Square

Prove: from the topic, focusing on m, n, p and q, there is a median theorem;

MQ=NP= 1/2DE，MQ//NP//DE

MN=PQ= 1/2AF，MN//PQ//AF

From (2) we know that AF=DE, then MQ=NP=MN=PQ.

And AF is perpendicular to DE, then all corners of the quadrilateral are right angles.

It is proved that quadrilateral is square.