f.2maths~DEDUCTIVE GEOMETRY

Are you sure it is BE//ED? Lines drawing from a same point E can be parallel only if it is the same line.

2009-11-28 19:30:39 補充：
Angle EBD + angle BDE = 180 - angle BED = 180 - 90 = 90 degree (angle sum of triangle).
2 x angle EBD + 2 x angle BDE = 90 x 2 = 180 degree.
2 x angle EBD = angle ABD because BE is the angle bisector. Similarly, 2 x angle BDE = angle BDC,
therefore, angle ABD + angle BDC = 180 degree.
therefore, AB//CD ( complementary angle ).