It is to judge the parallelism according to the conditions, that is, the parallelism is unknown.

For example, the internal dislocation angles are equal, and two straight lines are parallel, conditional first and then parallel.

The isosceles angles are equal and the two straight lines are parallel.

The internal angles on the same side are complementary and the two straight lines are parallel.

Properties of parallel lines:

Since these two lines are parallel, this conclusion is reached.

For example, two straight lines are parallel and the internal dislocation angles are equal.

Parallel first, then draw a conclusion.

Two straight lines are parallel and have the same angle.

These two lines are parallel and complementary.