Answer:
![\angle 1\cong \angle 2](https://img.qammunity.org/2020/formulas/mathematics/high-school/1x3f55ekabvg9f1im96ylhivd5nnatiyrq.png)
Explanation:
We have to find the missing statement in the given proof.
Angle 1 is congruent to angle 3
Given
Line a is parallel to line b
Given
![\angle 1\cong \angle 2](https://img.qammunity.org/2020/formulas/mathematics/high-school/1x3f55ekabvg9f1im96ylhivd5nnatiyrq.png)
By using alternate exterior angles theorem.
![\angle 2\cong \angle 3](https://img.qammunity.org/2020/formulas/mathematics/middle-school/w6c5bpmpi4ootnu063ydc67gwi4q1wz9de.png)
Therefore,
![\angle 2\cong \angle 3](https://img.qammunity.org/2020/formulas/mathematics/middle-school/w6c5bpmpi4ootnu063ydc67gwi4q1wz9de.png)
By the transitive property of congruence.
So, we can conclude that lines e and f are parallel by the converse alternate exterior angles theorem.
Answer:
![\angle 1\cong \angle 2](https://img.qammunity.org/2020/formulas/mathematics/high-school/1x3f55ekabvg9f1im96ylhivd5nnatiyrq.png)