Page 119 - ICSE Math 4

Diameter = 2 × Radius
Also, radius of a circle is half the length of its diameter.

1
r = × d = d  2 or Radius = Diameter  2
2

Example 2: If the radius of a circle is 3 cm, what is its diameter?

Solu on: Radius = 3 cm

Diameter = 2 × Radius = 2 × 3 cm = 6 cm
Example 3: If the diameter of a circle is 10 cm, what is its radius?

Solu on: Diameter = 10 cm

1
Radius = × Diameter = Diameter  2 = 10  2 = 5 cm
2
Construction of a Circle Using a Compass

A compass has two arms—one arm with a pointed p and the other arm that has a hole
to insert and fi x a pencil ghtly.

Let’s see how to draw a circle of radius 2 cm using a compass.

Step 1: Take the compass and fi x a pencil in it. Stretch Remember
the arms of compass to measure 2 cm with Make sure that the
the help of a ruler. It is done by keeping the pointed tip of the
pointed p of the device at 0 cm mark of the compass and the pencil
ruler and the pointed end of the pencil at 2 cm remain at the same level
while drawing a circle.
mark.

Step 2: Mark a point O on paper where you want to
draw the circle.

Step 3: Keep the pointed end of the compass on point O and slowly move the arm
that holds the pencil around on the paper by holding the compass from the
top.

We will get a circle of radius 2 cm with O as the centre.

O 2 cm O 2 cm

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