(x+3) 2 (square)

23. The sum of the internal angles of the quadrilateral is 360, and the angle B+ angle D= 180, so the angle BAD+ angle BCD= 180.

Their half angle BAE+DCF=90.

It is also known that in the triangle ABE, the angle BAE+AEB = 90°.

Therefore, the angle DCF=AEB.

So, Angel ·FCE = AEB

So, parallel

Do you understand?