Question

∠adb ≅ ∠cdb
reason:
all right angles are
congruent
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given
reflexive property
vertical angles are
congruent
all right angles are
congruent
an angle bisector divides an
angle into two congruent
angles
parallel lines cut by a
transversal form congruent
alternate interior angles
parallel lines cut by a
transversal form congruent
corresponding angles

Explanation:

The statement $\angle ADB\cong\angle CDB$ is justified by the geometric principle that all right angles have a measure of $90^{\circ}$, and angles with equal measures are congruent.

Answer:

All right angles are congruent.