2.
[tex]\begin{gathered} \angle RAG\text{ and }\angle RET\text{ are vertical angles} \\ \Rightarrow\angle RAG\cong\angle RET \end{gathered}[/tex]
The reason for 2 is 'the angles are vertical'.
3. In the diagram below, we can identify the alternate interior angles more easily
Therefore, Therefore, the statements of the third step are
[tex]\begin{gathered} \angle AGR\cong\angle ETR \\ \text{and} \\ \angle GAR\cong\angle\text{TER} \end{gathered}[/tex]
Finally, the corresponding angles of triangles GRA and TRE are all congruent; thus, the two triangles are congruent because of the AA similarity criterion. The answer is AA similarity criterion. The length of the sides is not involved in the demonstration.
