Circle and disc. Practical measuring

The set of all points that satisfy the same condition is called a locus of points. For example, a circle is the set of all points of the plane lying at the same distance from a chosen point O; O is the centre of the circle. The distance from any point of the circle to the centre is the radius; a segment joining two points of the circle is a chord; a chord through the centre is a diameter, and it is the longest chord. A diameter consists of two radii, so d = 2r. The part of the plane bounded by a circle is called a disc. We draw circles with a compass; its point is sharp, so use it carefully and under an adult’s supervision.