1, circular area: S=πr? ，S=π(d/2)？ . (d is the diameter and r is the radius).

2. area of semicircle: s semicircle = (π r 2)/2. (r is the radius).

3. Ring area: s great circle -S small circle = π (r 2-r 2) (r is the radius of great circle and r is the radius of small circle).

4. Circumference: C=2πr or c = π d..(d is the diameter and r is the radius).

5. The circumference of a semicircle: d+(πd)/2 or d+π r (d is the diameter and r is the radius).

circle

This is a geometric figure. By definition, compasses are usually used to draw circles. The diameter and radius of the inner circle of the same circle are always the same, and the circle has countless radii and diameters. A circle is a figure with axial symmetry and central symmetry. The axis of symmetry is the straight line where the diameter lies. At the same time, the circle is a "positive infinite polygon" and "infinity" is just a concept. A circle can be regarded as a regular polygon composed of countless infinitesimal points. The more sides a polygon has, the closer its shape, perimeter and area are to a circle.