Page 173 - Start Up Mathematics_4

7. Place a thread along the boundary of the circle. Straighten
the thread and measure its length. The length of the thread is P Q
equal to the length of the boundary of the circle and is called
its circumference. A O B

8. Mark any two points on the circumference and join them. The
straight line that you get is called the chord of the circle. A circle R S
can have many chords. They are of different lengths. AB, RS and PQ → chords

Example 1: If the radius of a circle is 3 cm, what is its diameter?

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

Radius = Diameter ÷ 2
= 10 ÷ 2 = 5 cm

Drawing a circle using a compass

A compass has two arms. One arm has a pointed tip and the other has a hole to insert a
pencil to draw the circle.

1. Take a compass and place the pointed tip on the paper at a fixed place. This will be
the centre of the circle.

2. Open the other arm of the compass with a pencil in it. The
distance between the two arms will be the radius.
3. Hold the top of the compass and move the arm with the
pencil around. Take care that the pointed tip at the centre
does not move. Your circle is drawn.

You can measure the radius of the circle drawn by measuring
the distance between the centre and any one point on the
circumference using a ruler.

EXERCISE 13.1

1. Tick the correct answer.

(a) Neha drew a polygon having four equal sides. Which shape did she draw?
(i) rectangle (ii) triangle (iii) square

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