Two straight lines are parallel and have equal internal angles.
∵BC=CD∴∠CDB=∠CBD
Equilateral equilateral angle
Also: the vertical BD∴ triangle ADB is a right triangle.
E is the midpoint of AB, and ∴DE is the center line of the hypotenuse of a right triangle ∴AE=EB=DE.
In a right triangle, the center line of the hypotenuse is equal to half of the hypotenuse.
∵DE=EB∴∠EDB=∠EBD
Equilateral equilateral angle
∴∠CBD=∠BDE∴CB∥DE
Internal dislocation angles are equal and two straight lines are parallel.
Then: the quadrilateral CDEB is a parallelogram.
∵CD=CB∴ The parallelogram CDEB is also prismatic.
A pair of parallelograms with equal adjacent sides are prismatic.