What reason can be used to complete this proof? ∠1=∠3 because ∠1≅∠3. ∠1+∠2=∠ and ∠2+∠3=∠ ______________. ∠1+∠2=∠ because you can substitute ∠1 for ∠3. ∠=∠ by the transitive property of equality. So, ∠≅∠.