Answer:
∠2; vertical angle theorem; ∠3; transitive property of congruence
Step-by-step explanation:
∠2 and ∠1 are opposite angles that share only a vertex. This is the definition of vertical angles, and by the vertical angle theorem they are congruent.
Substituting ∠2 for ∠1 in our "given" statement, we now know that ∠2≅∠4.
∠4 and ∠3 are opposite angles that share a vertex. This makes them vertical angles, and the vertical angle theorem states that they are congruent.
The transitive property, algebraically, states "if a=b and b=c, then a=c." Applied to this proof, it states "if ∠2≅∠4, and ∠4≅∠3, then ∠2≅∠3."
