In quadrilateral ACBD, AC=AD and AB bisects ∠A (see figure). Show that △ABC≅△ABD. What can you say about BC and BD?
In ΔACB and ΔADB ,
Side AB= Side AB [common side]
∠CAB=∠DAB [definition of angle bisector]
side CA= side DA [Given]
Therefore, By SAS test, we say that ΔACB≅ΔADB
Thus, the corresponding sides of congruent triangles are equal.
Hence, BC=BD