triangle ADE and triangle BCE are both right triangles.....definition of perpendicular
length of AE = length of EB.....given
m angle ADE = m angle BCE.....given
triangle ADE is congruent to triangle BCE.....right triangle with leg and angle equal
length of DE = length of EC.....corresponding arts of congruent triangles
m angle EDC=m angle ECD.....base angles of an isosceles triangle