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.
  

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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.
  

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