Know the circle and its circumference

1. Definition of a circle: A circle is a plane figure surrounded by a curve.

2. Center of the circle: Fold a circular piece of paper twice, and the point where the crease intersects the center of the circle is called the center of the circle. As shown in the figure below, the center of the circle O. is generally represented by the letter O, and its distance to any point on the circle is equal. (Don't forget to mark the center of the circle with 0)

3. Radius: The line segment connecting the center of the circle and any point on the circle is called radius. Generally, it is represented by the letter R, as shown below, red line. Separate the two feet of the compass, and the distance between the two feet is the radius of the circle.

4. Diameter: The line segment whose two ends pass through the center of the circle is called diameter. Generally, it is represented by the letter D, and the blue line is shown in the figure below. The diameter is the longest line segment in a circle.

5. The center of the circle determines the position of the circle, and the radius determines the size of the circle. If the diameter is known, we have to divide the diameter by 2, change it into radius, determine the center of the circle, and then start drawing a circle. Draw a circle and give the radius. Radius r=? , gives the diameter scale diameter d=? Comparing the sizes of two circles is to compare the diameters or radii of two circles.

6. In the same circle or equal circle, there are countless radii and countless diameters. All radii and diameters in the same circle are equal.

7. In the same or equal circle, the diameter is twice as long as the radius, and the radius is half as long as the diameter. Represented by letters: d = 2r or r=d ÷ 2.

8. Axisymmetric figure: If a figure is folded in half along a straight line, the figures on both sides can completely overlap, and this figure is axisymmetric. The straight line where the crease lies is called the symmetry axis.

9. Rectangles, squares and circles are symmetrical figures, and they all have axes of symmetry. These figures are all axisymmetric figures.