1. Diameter
Radius

3. Let’s try some questions: Figure out the answer to the question in alphabetical order and then click to reveal the correct answer A C E
d = 6cm
r = ?
D = 15cm
r = ?
r = 7cm
d = ? B D
d = 10cm
r = ? F
r = 4cm
d = ?
r = 3.5cm
d = ?

4. Circumference (C)
Circumference of a circle is the distance around the circle. You can think of it as the PERIMETER of the circle.
Find an object that has a circular base, like a pop can. With a string, measure the distance around the circular base. Check the measurement against a ruler. Now measure the distance across (diameter) that same circular base with your ruler. Do you notice that the circumference (C) is just about 3 times the diameter (d) ?
That’s what Archimedes found out, way, way back in history.
So to shorten up this finding we would write C = 3Xd or C = 3d. This is good for an estimate, but the number was proven to be something more like
…AND ON AND ON…it never ends.. Use 3.14, or
the button on your calculator.
SO, it was decided to use a symbol instead of that big long number
and pi is that symbol. SO, now we instead use C = d

5. Let’s try some circumference examples
Let’s try some circumference examples. Calculate the circumference of the following questions and then click to reveal the correct answers.
d=8cm
C=??
d=8cm
C= d
C=(3.14)(8)
C=25.12cm
d=10cm
C=??
d=10cm
C= d
C=(3.14)(10)
C=31.40cm
r=3cm
d=2r
d=(2)(3)
d=6cm
d=6cm
C= d
C=(3.14)(6)
C= cm
r=3cm
C=??

6. Area of a Circle Diameter (d) Radius (r) What is pi? 3.14
Remember, way, way back in time, Archimedes discovered that the distance around a circle (circumference) is approximately three times as long as the distance across. (diameter) We now know that it is more accurately an irrational number that we now recognize as pi. We usually use 3.14 instead of using all the decimals that never end.
What is pi?
3.14
What is the definition of the radius?
The distance from center of circle to any point on it.
Therefore, to find the area of a circle, you need to know this formula:
A = r2
Diameter (d)
REMEMBER
C= d OR C= r
Radius (r)

7. Try these questions. USE BEDMAS A= r2 = 3.14 x 32 = 3.14 x 9
= cm2
r = 3 cm
Remember to multiply the square first!
USE BEDMAS

8. Try this one too d=2r and r=d/2
No radius is given, therefore you must figure out the radius by dividing the diameter in half.
A= r2
= 3.14 x 1.82
= 3.14 x 3.24
= cm2
d = 3.6 cm
d=2r and r=d/2