A proof always starts with the givens
The 4 th statement is SAS, so we know in the statements 1,2,3 we need to show 2 sets of sides are equal and 1 set of angles
statement 1. shows and AB = BC so we have a set of sides
statement 3 shows a set of angles are equal
We need statement 2 to show that another set of sides are equal
For side angle side to work, the angle must be included ( between the two sides)
We have AB and angle ABD so we need BD = BD
BD = BD by the reflexive property