. WHY?
WHY?
if you will ONLY read well the post of Mr. PLATO, you almost there.
observe that, angle DAE = angle DAB - angle EAB = 90 - 60 = 30 degrees
but angle DAE is isoceles triangle.
Sum up the angles of triangle DAE, here we go
180 = (angle DAE) + (angle ADE) + (angle AED),
Therefore ....