Question

1. a || b if...

∠2≅∠3
∠2≅∠4
∠3≅∠5
∠4

Explanation:

Step1: Recall parallel - line postulates

When two lines are cut by a transversal, if alternate - interior angles are congruent, then the two lines are parallel. In the given figure, \(\angle3\) and \(\angle5\) are alternate - interior angles.

Step2: Apply the parallel - line condition

If \(\angle3\cong\angle5\), then by the alternate - interior angles postulate, \(a\parallel b\).

Answer:

\(\angle3\cong\angle5\)