Area and Circumference of a Circle

Area and Circumference of a Circle

Vocabulary

• The diameter (of a circle) is a chord (a line from one edge of a circle to another) that contains the center of a circle. Simply put: A line all the way across the circle.

• The radius (of a circle) is half the diameter or a line from the center of the circle to an point on the circle.

• The circumference (of a circle) is the distance around the circle (perimeter).

• The area (of a circle) is the measure of the interior region of a 2-Dimensional figure

Formulas for Circumference

 $C=2\pi r$  
  $C=\pi d$  (Know it but we won't really use it much.)

Using the Formula

• Enter the formula (without the "C=" in Desmos.

• Click enter/return.

• Next you have to tell Desmos what the variable is. In this formula, the only variable is r (radius)

• Type "r="

Using the Formula

• Take a look at the problem. Find the radius.

• Remember, if it gives you the diameter, you have to divide it by 2

• Type that number next to "r="

Let's look at an example.

Find the circumference of the circle with a radius of 8.

Circumference Example

• Enter the formula (without the "C=" in Desmos.

• Click enter/return.

• Next you have to tell Desmos what the variable is. In this formula, the only variable is r (radius)

• Type "r="

Circumference Example

• Take a look at the problem. Find the radius.

• In this example, the radius is 8.

• Type 8 next to "r="

Circumference Example

• AMAZING! Desmos does the work for you.

• Round as necessary.

• If you are working on a multiple choice assignment, look for the answer that is really close to what you get.

•  We are using $\pi$ , but some assignments may use 3.14 so you answer might not be exactly the same.
  

Fill in the Blank

You Try It!




Find the circumference. (Round to the nearest tenth.)

Fill in the Blank

Try another one.




Find the circumference. (Round to the nearest tenth.)

*Remember to use the radius...

Formula Area

 $A\ =\pi r^2$  

Using the Formula

• Everything works EXACTLY like it did when finding the circumference. You just use the area formula.

• Enter the formula (without the "A=" in Desmos.

• Click enter/return.

• Next you have to tell Desmos what the variable is. In this formula, the only variable is r (radius)

• Type "r="

Using the Formula

• Take a look at the problem. Find the radius.

• Remember, if it gives you the diameter, you have to divide it by 2

• Type that number next to "r="

Let's look at an example.

Find the area of the circle with a radius of 8.

Area Example

• Enter the formula (without the "A=" in Desmos.

• Click enter/return.

• Next you have to tell Desmos what the variable is. In this formula, the only variable is r (radius)

• Type "r="

Area Example

• Take a look at the problem. Find the radius.

• In this example, the radius is 8.

• Type 8 next to "r="

Area Example

• Again, Desmos does the work for you.

• Round as necessary.

• If you are working on a multiple choice assignment, look for the answer that is really close to what you get.

•  We are using $\pi$ , but some assignments may use 3.14 so you answer might not be exactly the same.
  

Fill in the Blank

You Try It!




Find the area. (Round to the nearest tenth.)

Fill in the Blank

Try another one.




Find the area. (Round to the nearest tenth.)

*Remember to use the radius...