We are given that ABCD is a square. If we consider triangle AEB and triangle AED, we see that side is congruent to side AD because sides of a square are congruent. We know that side AE is congruent to side AE by using the . Finally, we know that side DE is congruent to side because the diagonals of a square bisect each other. Therefore, triangle AEB is congruent to triangle AED by congruency. We see that angle AED and angle AEB are a linear pair, and congruent by CPCTC. Thus, the measure of these angles will be 90°, and diagonal AC is perpendicular to diagonal BD by the .