The reason why any angles coming out from <em>D</em> and <em>C</em> are congruents is because of the <em>geometry coungruence's concept</em> of 2 items having the same pattern, shape or form and size as the mirror image of the other.
2 angles would be congruent by being the same angle in degrees or radians.
Thereby any angle coming out from <em>C</em> as vertex would be congruent with any other vertex point angle mirroring the resulting figure shaped by the radiants or degrees of such angle.
The base angles C and D are congruent because this is an isosceles trapezoid. In addition, angle A = angle B because this is another pair of base angles (imagine rotating the figure 180 degrees to make segment AB the base)
