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A circle is a set of points which are all a certain distance from a fixed point known as the centre. A line joining the centre of a circle to any of the points on the circle is known as a radius.
The circumference of a circle is the length of the circle. The circumference of a circle = 2 × π × the radius.
The red line in the second diagram is called a chord. It divides the circle into a major segment and a minor segment.