- AC is congruent to itself
- AC bisects angle BAD, so that angles BAC and DAC are congruent
- Both angles B and D are right angles, so they are congruent
This means triangles BAC and DAC are a pair of angle-angle-side triangles, so they are congruent to one another. Corresponding parts of congruent triangles are congruent, so the line segments BC and DC are congruent and have the same length.
Then
