1. How to calculate the area of a circle.
It’s as easy as pi.

2. How in the world would you find the area of a circle?

3. Remember, area is always measured in square units.

4. Remember that the area of a rectangle is length x width because you’re calculating the total number of squares inside of the rectangle. 2 4

5. That’s fine and dandy, but a circle is not a polygon
That’s fine and dandy, but a circle is not a polygon. It does not have straight sides; it has curves.

6. How are we going to get around these curves?

7. Imagine chopping up the circle as if it were a pizza.

8. Now, let’s rearrange our “pizza” into another shape.

16. PRESTO!

17. Great Mr Loo! But what in the world is this?

18. Area of a Circle Discovery
Here is what we just did:
We labeled everything.
Area = base times
the height. r r
radius
Circumference is
the distance
around the circle.
We cut our circle
in half making two
pieces.

19. Area of a Circle Discovery
Watch what happens when we use a circle with 16 triangles instead of using a circle with 8 triangles. r r
Notice that with more triangles the figure begins to really resemble a parallelogram. Area of a parallelogram is base times height.

20. The area of a circle
Use π = 3.14 to find the area of the following circles:
2 cm
10 m
A = πr2
A = πr2
= 3.14 × 22
= 3.14 × 52
= cm2
= 78.5 m2
23 mm
78 cm
A = πr2
A = πr2
= 3.14 × 232
= 3.14 × 392
= mm2
= cm2

21. Take π = 3.14 Calculate the area of a circle with radius 1.2 cm.
(b) What is the radius of a circle with an area of 66 cm²?
Take π = 3.14
Answer
The radius is 1.2 cm
(a) A = π r ²
A = π x r x r
A = 3.14 x 1.2 x 1.2
A = cm²
A = 4.52cm² (2dp)
A = π r ²
66 = π x r ²
66 = 3.14 x r ²
r =  (66 ÷3.14)
r = 4.58cm (2dp)