I. Understanding:

1, which means:

When a line segment rotates once around its fixed end on a plane, a closed curve will appear at the other end, which is called a circle.

2, compass drawing:

(1) Sets the distance between feet.

(2) Fix the foot on a point with a needle tip.

(3) Turn your feet once with a pencil.

3. Name of each part:

(1) The center of the circle determines the position of the circle.

(2) The radius determines the size of the circle.

(3) The diameter is twice the radius (the same circle. )

Second, the circumference of the circle:

1, derivation of perimeter formula.

2. Area formula.

3. The area of the ring.

Expand knowledge:

First, the circle introduction:

A circle is a geometric figure, which refers to the set of all points on a plane with a constant distance from a fixed point. This given point is called the center of the circle. The distance as a fixed value is called the radius of a circle. When a line segment rotates once around one of its endpoints on a plane, the trajectory of its other endpoint is a circle. There are countless circles in diameter; A circle has countless axes of symmetry.

The diameter of a circle is twice the radius, and the radius of a circle is half the diameter. When drawing a circle with a compass, the point where the needle tip is located is called the center of the circle, which is generally represented by the letter O. The line segment connecting the center of the circle with any point on the circle is called the radius, which is generally represented by the letter R. The length of the radius is the distance between the two corners of the compass. The line segment passing through the center of the circle and with both ends on the circle is called the diameter, which is generally represented by the letter D.

A circle is a curved figure on a plane and an axisymmetric figure. Its axis of symmetry is the straight line where the diameter lies, and the circle has countless axes of symmetry.

Second, the concept of circle:

1, the set of points whose distance to a fixed point is equal to a fixed length is called a circle. This fixed point is called the center of the circle and is usually represented by the letter "O".

2. The straight line connecting the center of the circle and any point on the circumference is called radius, which is usually represented by the letter "R".

3. The line segment with two ends on the circumference passing through the center of the circle is called the diameter, which is usually represented by the letter "D".

4. A line segment connecting any two points on a circle is called a chord. In the same or equal circle, the longest chord is the diameter.

5. The part between any two points on a circle is called an arc. An arc larger than a semicircle is called an optimal arc and is represented by three letters. An arc smaller than a semicircle is called the lower arc and is represented by two letters. A semicircle is neither an upper arc nor a lower arc.