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.

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

• The coloured part inside is the area of a circle. (Figure: 2).
Figure: 2

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

• 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 $$2·5 = 10$$ ($$cm$$).