Page 144 - Math Skill - 4

142 Maths

Absorbing Facts
� Every diameter is a chord, but every chord is Fact-o-meter
not a diameter. A circle has many diameters. All

� The diameter is the longest chord in any circle. diameters have the same length.
� A circle has many radii, chords and diameters So,
but it has only one centre. length of MN = length of PQ.

Circumference

The length of the boundary of a circle is called its circumference. We cannot
measure the circumference of a circle with ruler though it is the length of the O Circumference
boundary of a circle.

Drawing a Circle of Given Radius

To draw a circle of radius (say 4 cm), we follow these steps:
Step 1: Open the legs of the compass to 4 cm using ruler as shown here:

Fact-o-meter

S
Q

R
P

PQ is chord. 3 RS is chord. 3
PQ is diameter. 7 RS is diameter. 3

Step 2: Mark any point O on the plane of the paper and place the tip
of steel end at it.

Step 3: Hold the head of the compass steadily and rotate it such that 4 cm O
the pencil end touches the paper.

Thus, a circle of radius 4 cm is drawn.

Relation between Radius and Diameter

We can see that AB is a diameter of the circle O.
Here, AB = AO + OB
AO and OB are the radii of the circle O. A B

So, AB = radius + radius. Here, AB is the diameter. O
Thus, diameter = radius + radius = Twice of the radius = 2 × radius
diameter
diameter = 2 × radius or radius = 2