Circle

A circle is a flat geometric figure made up of all the points in a plane that are at the same distance from one fixed point, called the center. This distance is the radius. The segment passing through the center and joining two points on the circle is called a diameter, and its length is twice the radius.

The distance around the circle is its circumference. A circle has an important symmetry: every line passing through its center is an axis of symmetry, and rotating the figure around its center leaves it looking unchanged. Unlike polygons such as squares or triangles, a circle has no straight sides or corners. Its boundary is a continuous curve.

Circles are found throughout everyday life, from wheels and plates to clocks, buttons and coins. Their shape is also essential in many areas of science, engineering and technology. In geometry, the circumference of a circle with radius r is calculated using 2πr, while its area is πr². The number π, read “pi,” is a mathematical constant whose value is approximately 3.14159.

A circle can also be divided into sectors, arcs and semicircles, making it useful for studying angles, lengths and areas. Because every point on its boundary is equally distant from the center, the circle provides a simple way to explore ideas such as radius, diameter, symmetry and rotation.