1, connect two points with a ruler to get a line segment; Extending one end of the line segment indefinitely can get a ray; Extend both ends of a line indefinitely and you can get a straight line. Line segments and rays are both parts of a straight line. A line segment has two endpoints and its length is limited. A ray has only one endpoint, a straight line has no endpoint, and both rays and straight lines are infinitely long.

2. Two rays from a point form an angle. The size of the angle is related to the size of both sides, and has nothing to do with the length of the sides. The unit of measurement of angle size is ().

3. Classification of angles: angles less than 90 degrees are acute angles; An angle equal to 90 degrees is a right angle; An angle greater than 90 degrees and less than 180 degrees is an obtuse angle; The angle equal to 180 degrees is a flat angle; An angle equal to 360 degrees is a fillet.

4. Two straight lines intersecting at right angles are perpendicular to each other; Two straight lines that do not intersect in the same plane are parallel to each other.

5. A triangle is a figure surrounded by three line segments. Each line segment constituting a triangle is called an edge of the triangle, and the intersection of every two line segments is called the vertex of the triangle.

6. Triangle can be divided into acute triangle, right triangle and obtuse triangle according to angle; According to different sides, it can be divided into equilateral triangle, isosceles triangle and arbitrary triangle.

7. The sum of the internal angles of a triangle is equal to 180 degrees.

8. In a triangle, the sum of any two sides is greater than the third side.

9. In a triangle, there is at most one right angle or at most one obtuse angle.

10, line

(1) A straight line without an endpoint; Infinitely long; You can draw countless lines after one o'clock, and only one straight line after two o'clock.

(2) ray, which has only one endpoint; Infinitely long.

(3) A line segment has two endpoints, which are part of a straight line; Limited length; In the connection between two points, the segment is the shortest.

(4) Parallel lines, two straight lines that do not intersect in the same plane are called parallel lines. The vertical lines between two parallel lines are equal in length.

(5) Vertical line, when two straight lines intersect at right angles, these two straight lines are called perpendicular to each other, one of them is called the vertical line of the other straight line, and the intersection point is called vertical foot. The length of a vertical line drawn from a point outside a straight line is called the distance from that point to the straight line.