A circle is a line drawn at a fixed distance from a point. The point is called the centre, and the line that is drawn a fixed distance from it is called the circumference.
The distance from the centre to the circumference is the radius.
A line joining one part of the circumference to another part of the circumference is known as a chord. A chord that goes through the centre of a circle is a diameter, and has twice the length of the radius.
A circle has a radius of 8cm. What is the longest chord that can be drawn on the circle?
The longest chord must go through the centre of the circle. This is the diameter; the diameter is twice the value of the radius. 2 x 8 = 16cm.
A line is drawn from the centre of a circle to its circumference. The diameter of the circle is 6cm. What is the length of the line?
The line from the centre of a circle to its circumference is the radius, and the radius is half the diameter. 6 ÷ 2 = 3cm.