Lesson 4

Area and circumference of a circle

  
The length of thr black line forming the circle is called the circumference. 

It is shown by the symbol C. 

The diameter is the blue line shown above. It is a line drawn through the center of a circle and is shown by the symbol d 

The radius are the red lines shown above. The radius of a circle is the distance from the center to the edge of a circle. It is ¹/₂ of the diameter and is shown y the symbol r. 

If you measure the circumference and the diameter of a circle and divide the circumference by the diameter, you get the number Pi, which is about 3.14. 

 Pi is often shown by the Greek letter π  .

The ancient Greeks used the symbol π   to indicate the ratio between the circumference and diameter of a circle. If your calculator does not have the symbol π   , you can use the rounded off number 3.14 for π .

Some formulas that are good to know and understand.

Symbol	Name	Formula
π	Pi
d	Diameter		and
C	Circumference
r	Radius
A	Area

Some examples

Example 1.

Find the diameter.     d = 2 × 3cm = 6cm

Example 2.

Find the circumference.     C = d × pi = 6cm × 3.14 = 18.8 cm

Example 3.

    Find the diameter.   

d= 18.8 ÷ 3.14 = 6 cm

Example 4.

    The area of the circle is:   

A = 5m × 5m × 3.14 78.5m²  (Note: the symbol means about the same as).

In the diagram above, the area of the small yellow square is a quarter of the area of the big square. 

You can see that the area of the circle is less than that of the square since the circle fits inside it.  You can also find the area of the circle by multiplying the area of the small square by Pi:   

Practice these methods and then try Quiz 4 on Area. Remember to use the Checklist to keep track of your work.