To complete the proof in step 3, we need to establish that triangles ΔADB and ΔCDB are congruent based on the information given in the preceding statements.
The triangles share the segment BD (which is denoted as BD ≅ BD, using the Reflexive Property of Congruence). We also know that AB ≅ CB (stated in step 1) and ∠ABD ≅ ∠CBD (also from step 1).
With two sides and the included angle congruent between two triangles, we can apply the Side-Angle-Side (SAS) Triangle Congruence Theorem.
Thus, the best reason to complete the proof in step 3 is:
Side-Angle-Side Triangle Congruence Theorem