Unit 16 Section 1 : Introduction to circles

In this section we look at the parts of a circle and what they are called.

The edge of a circle is called the circumference. The point right in the middle is called the centre.

 

There are three different types of straight lines we can draw on a circle:

  • A chord is any straight line which is drawn between two points on the circumference.
     
  • A diameter is a chord that passes exactly through the centre of the circle.
     
  • A radius is a straight line drawn from the centre of the circle to the circumference.
     

When you draw two lines from the centre of the circle to different points on the circumference, the shape formed by the two radii and the edge of the circle is called a sector.

The curved part of the circle between the two points on the edge is called an arc.

 

In later sections we will look at how the length of the radius and diameter of a circle are linked to the length of the circumference and the area of the whole circle. Below are some questions to help you practise remembering which part of the circle is which.

When you draw a chord on a circle, you split the circle into two segments.

The smaller one is called the minor segment.

The larger one is called the major segment.

If the chord is a diameter, then the two segments are exactly the same size; they are both semicircles.

 

Exercises

Work out the answers to the questions below and fill in the boxes. Click on the Click this button to see if you are correct button to find out whether you have answered correctly. If you are right then will appear and you should move on to the next question. If appears then your answer is wrong. Click on to clear your original answer and have another go. If you can't work out the right answer then click on Click on this button to see the correct answer to see the answer.

Question 1
The diagram below shows a circle with various straight lines on it.
Work out the type of each line below (the line goes between the two letters shown).

O is the centre of the circle.

(a) O A

(b) A B

(c) B C

(d) O D

(e) C D

(f) A C

(g) A D

Question 2
This circle has been divided up into various different regions.

Work out what shape each region is.

(a) Region A is

(b) Region B is

(c) Region C is

(d) Region D is

(e) Region E is

(f) Region F is


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Produced by A.J. Reynolds August 2008