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?

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 =

• 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.

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.

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.

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

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