MJ2

Ch 11.6 – Area of Circles

Bellwork

Find the area of the triangle to the nearest tenth. Use the formula

A = ½bh

26.1 cm

29.3 cm

30.5 cm

Bellwork Solution

A = ½bh

26.1 cm

29.3 cm

30.5 cm

A = ½(764.73)

A = ½(26.1)(29.3)

A = 382.365

A = 382.4 cm2

1. Write the formula

2. Substitute

3. Do the math

Assignment Review

Text p. 491 # 8 – 15

Note: Your bellwork is due today…Please pass it forward…

Before we begin

Please take out your notebook and get ready to work…

Continuing with the theme of area, in today’s lesson we will look at how to calculate the area of a circle

Of course, you will use a formula and the formula method to demonstrate what you know….

Objective

Students will calculate the area of circles

Vocabulary

In a previous lesson we defined the parts of a circle…so this should be a quick review

Diameter – the distance across a circle through the center

Radius – the distance from the center to any point on the circle

Pi (3.14) – the numerical expression that represents the relationship between the diameter and circumference of a circle

Circles

To calculate the area of a circle use the formula

A = ∏r2

Be careful here….the formula calls for r.

If you are given d, you have to convert it to r

We all remember that r is ½ of d

Example #1

A = ∏r2

6.5 m

A = (3.14)(6.5)2

A = 3.14(42.25)

A = 132.665

A = 132.7 m2

1. Write the formula

2. Substitute

3. Do the math

Your answer will be squared because you are measuring 2 dimensions

Example #2

A = ∏r2

16 m

A = (3.14)(8)2

A = 3.14(64)

A = 200.96

A = 200.96 m2

1. Write the formula

2. Substitute

3. Do the math

Since you are given the diameter you have to convert it to radius to use in the formula

Your Turn

In the notes section of your notebook draw and label the circle and then calculate area to the nearest tenth using the formula

A = ∏r2

6 yd4 cm

1. 2.

Your Turn Solution #1

A = ∏r2

4 cm

A = 3.14(4)2

A = 3.14(16)

A = 50.24

A = 50.2 cm2

1. Write the formula

2. Substitute

3. Do the math

Your Turn Solution #2

A = ∏r2

A = 3.14(3)2

A = 3.14(9)

A = 28.26

A = 28.3 yd2

1. Write the formula

2. Substitute

3. Do the math

6 yd

Summary

In the notes section of your notebook summarize the key concepts covered in today’s lesson

Today we discussed: Calculating area of circles What method do you use? What do you do if they give you the

diameter?

Assignment

Text p. 494 # 11 – 20Reminder

This assignment is due tomorrow I do not accept late assignments Draw and label the picture and use the

formula method Answers only will not be accepted for a

grade