1. Two straight lines that do not intersect in the same plane are called parallel lines, or they are parallel to each other. Note: A∑b is pronounced as: A and B are parallel.

2. When two straight lines intersect at right angles, they are said to be perpendicular to each other, one of which is called the perpendicular of the other, and the intersection of these two straight lines is called the vertical foot. Note: a⊥b reads: A is perpendicular to B.

3. The shortest vertical line drawn from a point outside a straight line is called the distance from the point to the straight line.

4. The line segments perpendicular to two parallel lines are equal in length. Or: the distance between two parallel lines is equal everywhere. You can draw a straight line by taking a point (or an outer point) on the straight line as a vertical line.

5. In the same plane, two straight lines parallel (or perpendicular) to the same straight line are also parallel to each other.

6. Draw a vertical line from one point on one side of the parallelogram to the other. The line segment between this point and the vertical foot is called the height of the parallelogram, and the side where the vertical foot is located is called the bottom of the parallelogram.

7. Hold the two opposite corners of the rectangle with both hands and pull them in opposite directions. You can draw the rectangle into a parallelogram with different shapes, but the perimeter remains the same.

8. Characteristics of parallelogram: easy to deform. Such as retractable doors and elevators.

9. Parallelogram and trapezoid have countless heights.

10, isosceles trapezoid is called isosceles trapezoid. Features: Two waists are equal, and two bottom angles are equal.

A trapezoid with a right angle is called a right trapezoid. Features: A waist is the height of a trapezoid.

12. Take any point from the upper bottom of the trapezoid and draw a vertical line to the lower bottom. The line segment between this point and the vertical foot is called the height of the trapezoid.

13. Two identical triangles can be combined into a parallelogram. Two identical trapezoids can be combined into a parallelogram. Two identical right-angled trapezoids can be combined into a rectangle or parallelogram.

14, rectangle is a special parallelogram, and square is a special parallelogram. A square is a special rectangle.

15, the sum of three internal angles of a triangle is 180, and the sum of four internal angles of a quadrilateral is 360.

16, quadrilateral summary: two groups of quadrilaterals with parallel opposite sides are called parallelograms; A quadrilateral with only one set of parallel sides is called a trapezoid. An isosceles trapezoid is called an isosceles trapezoid. A trapezoid with right angles is called a right trapezoid. A quadrilateral with four corners at right angles is called a rectangle. A quadrilateral with four right angles and four equal sides is called a square.