Understanding of 1 and circle
Definition of circle: A circle is a plane figure surrounded by a closed curve on a plane.
Center: The center of a circle is called the center. It is represented by the letter "O".
Radius: The line segment connecting the center of the circle and any point on the circle is called the radius, which is represented by the letter "R".
Diameter: The line segment passing through the center of the circle with both ends on the circle is called diameter, which is represented by the letter "D".
The center of the circle determines the position of the circle, and the radius determines the size of the circle.
Equal circle: circles with equal radii are called equal circles, and equal circles can be completely overlapped by translation.
Concentric circles: Two circles with coincident centers and unequal radii are called concentric circles.
Properties: All radii are equal and all diameters are equal in the same circle. In the same circle, there are countless radii and diameters.
2, the circumference of the circle
Circumference: The length of a curve around a circle is called circumference. It is represented by the letter C. The circumference formula of a circle is: c = π d = 2 π r.
Pi: (1) We call the ratio of the circumference to the diameter of a circle Pi, which is expressed by the letter π. (2) Pi is an infinite acyclic decimal. In the calculation, π≈3. 14 is taken. (3) The first person in the world to calculate pi was Chinese mathematician Zu Chongzhi.
Variation law: the radius is enlarged many times, the diameter is also enlarged many times, and the circumference is also enlarged by the same times as the radius and diameter.
3, the area of the circle
Definition: The size of the plane occupied by a circle is called the area of the circle. It is represented by the letter s.
Area formula: S=πr? .
4. Fan-shaped
Definition of sector: The part between any two points on a circle (such as point A and point B) is called an arc (pronounced as arc AB), and the figure surrounded by an arc and two radii passing through the two ends of this arc is called a sector.
Central angle: the angle of the vertex at the center of the circle is called the central angle. In the same circle, the size of the sector is related to the size of the central angle.
Sector area: S=πr? ×n/360(n represents the degree of the central angle of the sector).
The above is the mind map of the first volume of the sixth grade mathematics circle.