The most important parallel lines are the ipsilateral angle and the internal angle, and the others are their extension lines. For example, the so-called "trumpet" is mainly the following graphics:

The relationship between ∠ 1, ∠2 and ∠3 can be easily deduced with the knowledge we have learned.

When studying, it is mainly to learn the basic knowledge well and learn it solidly, so that all the expansion is self-evident.

If all your classmates know and you don't, it means that the teacher didn't tell you, but you didn't hear it.