(Rectangular, trapezoidal, etc. Are all plane figures surrounded by several line segments?)

2. Radius: The line segment with one end in the center and one end on the circle is called radius. In the same circle, there are countless radii, all equal.

3, diameter: through the center of the circle, the line segment with both ends on the circle is called diameter. The same circle has countless diameters, all of which are equal.

The diameter of the same circle is twice the radius. (d=2r，

r=d÷2)

4. A circle is an axisymmetric figure with numerous symmetry axes, and the symmetry axis is the diameter.

5. The center of the circle determines the position of the circle, and the radius determines the size of the circle.

6. The largest circle in a square. The two are related: side length = diameter.

7. The largest circle in a rectangle. The two are related: width = diameter.

8. The diameter is the longest line segment in a circle.

1 1, the circumference of a semicircle is equal to half the circumference plus a diameter.

14, the area of a semicircle is half of a circle. S half =πX

Square of r ÷2

15. Compare two circles, the multiple of radius = the multiple of diameter = the multiple of perimeter, and the multiple of area = the multiple of radius = 2 times.

16, the area of the circle is the largest in the plane graphics with equal perimeters; In a plane figure with equal area, the circumference of a circle is the shortest.

17. A triangle with three vertices on a circle and a diameter on one side must be a right triangle.

By applying this rule, the diameter and center of a circle can be found.

(1) Draw a right angle with a point on the circle as the vertex.

(2) Connect two sides of an angle with two intersections of a circle, and this line is the diameter.