Given a quadrilateral ABCD and the squares constructed externally on each side, then the two line segments between the centers of opposite squares are congruent and are perpendicular.
	     
 

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			Quadrilateral
 
			
			Square
              
		                
	  Triangle
 
              
		                
				
				Perpendicular lines
 
               
		
			
			Congruence
			
			Classical Theorems
 
               
		
			Dynamic Geometry
      	van Aubel's theorem proof
			        
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