Theory:

Familiarize yourself with the mathematics tool - the Compass.
 
Compass can look different, but all whips have two legs - a sharp-pointed pin and a pencil.
 
Compass can be used to draw a circle.
 
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If you mark a small mark or dot on a piece of paper and place the sharp crank leg at that point, you can draw a circle. The dot is called the center of the circle, and we usually denote by the capital letter \(O\).
The segment joining center \(O\) of a circle to a point on the circle (point A in the drawing) is called the radius.
The figure shows that the radius of the circle \(OA\) is \(5\)\(cm\) (Figure: 1).
Compass is needed - with the feet of the cruiser measured to the desired distance to draw a circle with a given radius.
 
Figure: 1
  
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  • The coloured part inside is the area of a circle. (Figure: 2).
Figure: 2
  
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The segment passing through the center of the circle and joining the two points of the circumference is called the diameter.
  • The diameter is \(EF\) (Figure: 3).
Figure: 3
  
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  • The segments \(AO, BO, CO, EO, FO\) are the radii of the circumference (Figure: 3).
The radii of the circle are equal in length.
\(AO = BO = CO = EO = FO\)
The diameter of the circle is \(2\) times longer than the radius.
If the radius of the circle is \(5\)\(cm\), then the diameter is \(2·5 = 10\) (\(cm\)).