All sides of a square re equal and opposite sides are parallel to each other. Assuming that E is the point where the diagonals intersect (since we are not told in the question), then <span>AEB ≅ DEC. By SAS, 2 sides are equal. That is AE = EC and DE = EB. The triangles share a common angle. That is <DEC=<AEB
The property used t</span><span>he diagonals bisect each other.</span>