In ∆abc shown below, ab is congruent to bc.

given: ab = bc. prove:

prove: the base angles of an isosceles triangle are congruent.

the two-column proof with missing statement proves the base angles of an isosceles triangle are congruent.

I found the additional attachment of this problem.

The question was Which statement can be used to fill in the numbered blank space.

The number blank space is number 3 under the Statement column. The Reason column stated that number 3 is Reflexive property.
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The missing statement is BD ≡ BD

The above triangle can be divided into two equal triangle when we cut it along the line BD. Thus, the reflexive property of congruence.