Names of parts of a circle: a point at the center of the circle is called the center of the circle (O); The line segment connecting the center of the circle and any point on the circle is called radius (r); The line segment whose two ends pass through the center of the circle is called diameter (D); The length of the curve forming a circle is called the circumference of the circle (C); The size of a circle is called its area.

Calculation formula of circle: c=πd/

2πr

s=πr？

The calculation formula of semicircle is: c semicircle =πd÷2+d/πr+2r.

S semicircle =πr? ÷2