90
Drawing a circle
Textbook: Part 3, lesson 90, pp. 67–69
GoalDraw a circle of a given radius with a compass; circles with the same centre.
New words
compass · sirkulcentre · markazradius · radiusneedle point · ignali uch
Explanation
A circle is drawn with a compass. 1) Mark the centre. 2) Open the compass so the distance between its needle and pencil points equals the radius. 3) Without changing the opening, put the needle on the centre and draw the circle with the pencil. If the diameter is given, open the compass to half of it.
Worked examples
For a circle of radius 4 cm, open the compass to 4 cm; its diameter is 8 cm.
Circles with the same centre have radii 5 cm and 3 cm: the ring is 5 − 3 = 2 cm = 20 mm wide.
Class activity
“Target”: children draw 5 circles with the same centre (radii 1, 2, 3, 4, 5 cm), colour the rings and measure the width of each ring.
Practice
1
To draw a circle with diameter 10 cm, how many centimetres should the compass be opened?
5
2
A circle has radius 4 cm. How many millimetres is its diameter?
80
3
Two circles with the same centre have radii 7 cm and 4 cm. How many millimetres wide is the ring between them?
30
4
A compass is opened to 6 cm. How many centimetres is the diameter of the circle it draws?
12
5
If 3 radii are drawn from the centre of a disc, into how many sectors is it divided?
3