Find the circumference of the following circles: (Write the formula that you will use.)

Find the perimeter: (Write the formula that you will use.)

How to calculate the area of a circle. Its as easy as pi.

Lets first make sure that we understand the difference between circumference and area.

The circumference of a circle is the perimeter of the circle.

Imagine that the circle is straightening itself out.

The length of this line segment is the circumference of the circle. 314 cm

The circumference is the same length as 3 diameters plus.14 of another diameter.

So, circumference = diameter x 3.14

Does this look familiar?

O.K., now its time to move forward with some new stuff.

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

Remember, area is always measured in square units.

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

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

How are we going to get around these curves?

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

Now, lets rearrange our pizza into another shape.

PRESTO!

Great! But what in the world is this?

Believe it or not, this is really our friend the parallelogram.

Rats! He always has an answer for everything.

Area = Base x Height Base Height

To find the area of the circle (which is now a parallelogram), we just need to multiply the Base by the Height. Base Height

Base Radius Wait a minute! The height of this parallelogram is really the radius of the circle.

1/2 of Circumference Radius Wait a minute! The Base is really 1/2 of the circumference.

1/2 of Diameter x Radius Wait a minute! The circumference is really Diameter x

Radius x Radius Wait a minute! 1/2 of a Diameter is really a Radius.

Base Height So if we multiply the Base x Height

We are really multiplying Radius x Radius x Radius x Radius

Practice Time!

1) Now lets try this formula. Find the area of this circle. 5 cm

5 x 5 x 3.14 = 78.5 square cm 5 cm

2) Find the area of this circle. 6 cm

6 x 6 x 3.14 = square cm 6 cm

3) Find the area of this circle. 9 cm

9 x 9 x 3.14 = square cm 9 cm

4) Find the area of this circle. 20 cm

10 x 10 x 3.14 = 314 cm 2 20 cm Make sure that you use the radius of the circle.

5) Find the area of this circle. 14 cm

7 x 7 x 3.14 = cm 2 14 cm Make sure that you use the radius of the circle.

6) Find the area of this circle. 22 cm

11 x 11 x 3.14 = cm 2 22 cm

Area = Radius x Radius x Its as easy as pi.

The Area and Perimeter of a Circle A circle is defined by its diameter or radius Diameter radius The perimeter or circumference of a circle is the distance around the outside The area of a circle is the space inside it The ratio of π (pi) π is an irrational number whose value to 15 decimal places is π = We usually say π3.14 The circumference is found using the formula C=π d or C= 2πr (since d=2r) The area is found using the formula A=πr 2