Key point: 1, two straight lines with only one common point are called intersection line 2. Master the definitions of adjacent complementary angles, vertex angles, vertical lines and vertical line segments.

3. There is one and only one straight line perpendicular to the known straight line after a point outside the straight line; 4. Among the point-to-point line segments on a straight line, the vertical line segment is the shortest. 5. Master the definitions of congruent angle, ipsilateral internal angle and internal dislocation angle. 5.2 Parallel lines and their properties.

Point: 1 Through a point outside the straight line, there is one and only one straight line perpendicular to the known straight line. 2. if the straight line is a//c and b//c, then a//b;

3. The same angle is equal and two straight lines are parallel; Internal dislocation angles are equal and two straight lines are parallel; The internal angles on the same side are equal, and the two straight lines are parallel; On the contrary, the same is true.