Either AD is parallel to BC or AB and DC are equal.
http://answers.yahoo.com/question/index?qid=200912...

Okay, I think I have it, and it was easier than I expected. Without loss of generality, set up one side along the yaxis with the midpoint at the origin. One corner of the quadrilateral
http://answers.yahoo.com/question/index?qid=201202...

a is the answer. Because vertical angles are = you can prove the opposite sides of the quadrilateral are = to each other. y= 2.6x, and. y+9.1= 4x3.5. Substitute 2.6x for y getting:
http://answers.yahoo.com/question/index?qid=200703...

Since all four sides are equal they must be 360/4 = 90 degrees. If a quadrilateral has all four sides equal it is a rhombus. However all four angles being 90 degrees doesn't guarantee
http://answers.yahoo.com/question/index?qid=201104...
