Because angle c is 90 degrees vertical BC and DF is vertical AC, angle DEC= angle DFC=90 degrees, so quadrilateral CFDE is rectangular. Because the bisector of angle A and angle B intersects with point D (DG is perpendicular to AB and perpendicular to vertical foot G when intersecting with point D) (because the distance from any point on the bisector of triangle to both sides is equal), so ...