These all start the same way:
1. AD ≅ CD Given
2. BD ⊥ AC Given
ok, now we're aiming for congruent triangles using SAS
3. BD ≅ BD Reflexive property
4. ∠ADB and ∠CDB are right Def perpendicular lines
5. ∠ADB ≅ ∠CDB are right Right angle congruence theorem (!)
6. ΔADB ≅ ΔCDB SAS
7. ∠ABD ≅ ∠CBD CPCTC
