Page 118 - ICSE Math 4

1. Take any round object like a bangle. Trace its boundary to get a circle.
2. Cut this circle along the boundary.

3. Fold the circle into half and press the fold ghtly.

4. Now open the fold. The crease that you get is called
the diameter of the circle. It divides a circle into two
A B
equal halves and passes through the centre of a circle. Diameter
Draw a line on the crease and mark it as A and B as

shown.

5. Fold the circle into half along a diff erent line. The new
crease that you get is also a diameter. The two diameters
meet at a point. This point is called the centre of the
circle. Mark it as O. O

Centre

6. The distance from the centre to any point
on the boundary of the circle is called the L
radius of the circle. All radii (plural of radius) O Radius O  Centre
A B AB Diameter
of a circle are equal. Draw a line from O to Diameter
OA, OB and OL  Radii
any point on the boundary of the circle and
mark the point as L.

• AB is a diameter of the circle. Diameter can be wri en as d.

• O is the centre of the circle.

• OA, OB and OL are the radii of the circle. Radius can be wri en as r.
7. Now, wind a thread or a string along the boundary of the bangle. Straighten it and
measure its length. The length of the thread or the string is equal to the length of
the boundary of the circle and is called its circumference.

Relationship between Diameter and Radius

Look at the fi gure given alongside. O divides PQ into OP and OQ
equally. P Q
O
Thus, PQ = OP + OQ, where PQ is the diameter, and OP and OQ are

the radii.
We can also write it as, d = r + r = 2r. Thus, diameter of a circle is twice the length of its
radius.

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