2. rounds

s=πrr

3. Definition of radius: The line segment from the center of the circle to any point on the circle is called radius.

4. Definition of diameter: The line segments on a circle whose two ends pass through the center of the circle are called diameters.

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

6, half

Formula: c=πr+2r

7, half

:s=πrr/2