# 2D shapes – circle

The circle is a basic two-dimensional shape in geometry that consists of all the points in a plane that are at a constant distance, called the radius, from a given point, called the center.

The circle is often symbolized by the letter “O” or “∘”.

Some properties of circles include:

• Circumference: The circumference is the distance around the circle. It is equal to 2π times the radius, where π (pi) is a mathematical constant approximately equal to 3.14.
• C = 2πr – where C is the circumference, π (pi) is a mathematical constant approximately equal to 3.14, and r is the radius of the circle.
• C = πd – if you know the diameter of the circle (the distance across the circle through its center), you can also use this formula, where d is the diameter of the circle.
• Diameter: The diameter is a line segment that passes through the center of the circle and has endpoints on the circle. The diameter is twice the radius.
• d = 2r – where d is the diameter of the circle, and r is the radius of the circle.
• Radius: The radius is a line segment that connects the center of a circle to any point on the circle. It is half the length of the diameter.
• r = C / 2π – where r is the radius, C is the circumference of the circle, and π (pi) is a mathematical constant approximately equal to 3.14.
• r = d / 2 – where d is the diameter of the circle.
• Area: The area of a circle is π times the square of the radius.
• Chord: A chord is a line segment that connects two points on the circle. The longest chord in a circle is the diameter.
• Tangent: A tangent is a line that intersects the circle at exactly one point. The tangent is perpendicular to the radius at the point of intersection.
• Segment: A segment in a circle refers to a part of the circle that is bounded by two points on the circle.
• Sector: A sector is a region of the circle that is bounded by two radii and an arc.
• Arc: An arc is a portion of the circumference of a circle. The length of an arc is proportional to the angle it subtends at the center of the circle.