Circles Intro

We're starting CIRCLES! As easy as a slice of pi!

​What are the parts of a circle?

The distance between the center of the circle and the edge is called the RADIUS

The distance from the edge of a circle, straight across the center, to the other edge, is called the DIAMETER.

The diameter is double the radius. The radius is half the diameter.

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If the radius of a circle is 5, what is the diameter?

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If the diameter of a circle is 4.8, what is the radius?

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What is the diameter?

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What is the radius of the circle?

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Pi is always the number 3.141592653589793238462...

It goes on forever, but in 7th grade, we will use 3.14 as an approximation for pi.

Pi - π

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π is approximately =

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• The perimeter of the circle

• You can find the circumference using the formula: C=πd

• Write this formula down for every circumference problem

​Circumference

Find the circumference

1. Write down the formula: C=πd

2. Replace pi with 3.14 in the formula: C=(3.14)d

3. Replace diameter in the formula: C=(3.14)(30.2)

4. Solve the equation: C=94.828 cm

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What's the circumference? Use 3.14 as pi.

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Poll

What word means perimeter of a circle

• Circumference

• Diameter

Find the circumference

1. Write down the formula: C=πd

2. Replace pi with 3.14 in the formula: C=(3.14)d

3. Turn the radius into a diameter:

   r = 14, so d = 14 x 2 = 28

4. Replace diameter in the formula: C=(3.14)(28)

5. Solve the equation: C=87.92 in

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What's the circumference. Use 3.14 for pi. Hint: you need the diameter.

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Find the diameter and radius

If you're given the Circumference, we can work backwards to find the diameter

1. Write down the formula: C=πd

2. Replace pi with 3.14: C=(3.14)d

3. Replace C with the circumference: 40.82=3.14d

4. Divide both sides by 3.14 /3.14 /3.14

5. Divide your diameter by 2 13 = d

6. That's your radius 6.5 = r

​C=40.82

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What's the radius of a circle with a Circumference of 56.52

• How much space is inside a circle

• You can find the circumference using the formula: A=πr²

• r² means r x r, kind of like length times width.

​Area

Find the Area of a circle

1. Write down the formula: A=πr²

2. Replace pi with 3.14 in the formula: A=3.14r²

3. Replace radius in the formula: A=3.14(5)²

4. Square the radius: A=3.14(25)

5. Solve: A=78.5 cm²

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What's the area. Use 3.14 for pi.

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Find the Area of a circle

1. Write down the formula: A=πr²

2. Replace pi with 3.14 in the formula: A=3.14r²

3. Determine the radius

   d = 26, so r = 26/2 = 13

4. Replace radius in the formula: A=3.14(13)²

5. Square the radius: A=3.14(169)

6. Solve: A=530.66 cm²

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What's the area. Use 3.14 for pi. Hint: find the radius first

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Multiple Choice

What number represents π?

• 31.4

• 4.13

• 3.14

• 34.1

Multiple Choice

Does this picture show the diameter or radius of a circle?

• Diameter
• Radius

Multiple Choice

Find the circumference of the circle.

• 78.5 cm

• 15.7 cm

• 7.85 cm

• 19.6 cm

Multiple Choice

Find the circumference of the circle.

• 38.47 cm

• 10.99 cm

• 43.96 cm

• 21.98 cm

Multiple Choice

What is the radius of this circle?

• 18
• 10
• 9
• 8

Multiple Choice

Find the area of the circle

• 43.96
• 153.86
• 87.82
• 307.72

Multiple Choice

Find the area of the circle, giving your answer to 1 decimal place.

• 3.1cm²

• 12.6cm²

• 6.3cm²

• 50.3cm²

Multiple Choice

What is the area of this circle?

• 201.06 square meters

• 50.24 square meters

• 123.12 square meters

• 25.12 square meters

Multiple Choice

A circle has a diameter of 17 inches. What is the area?

• 26.69 in²

• 226.87 in²

• 72.25 in²

• 53.38 in²

Multiple Choice

What is the circumference of this circle ?

• 3.14 cm 
• 37.68 cm
• 24 cm
• 68.4 cm