Answer:
Red marking indicate the two objects are congruent by hypothesis.
Green numbers marks the step below:
1. Angles ABD and CBD are congruent by definition of angular bisector;
2. Side DB is in common between the triangles ABD and CBD;
3. Angles ADB and CBD are congruent since they're both right angles.
1,2,3 [tex]\implies[/tex] Triangles ADB and CBD are congruent by ASA, thus corresponding sides are congruent, in particular AD is congruent to CD, which means D is the midpoint of AC (q.e.d.)