Compare Perimeter: Circle (r=3cm) vs Square (side=7cm)

Perimeter Comparison with Circles and Squares

The radius of a circle is 3 centimeters.

The length of the side of a square is 7 centimeters.

Which shape has a larger perimeter/circumference?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:09 Which shape has a smaller perimeter?
00:13 Let's find the perimeter of the circle first.
00:16 The circle's radius is given in the problem.
00:20 We'll use the formula: Perimeter equals 2 times pi times the radius.
00:26 Now, let's replace the radius with the given number, and calculate the perimeter.
00:37 Substitute pi with three point one four. Let's compute the result.
00:48 This value is the circle's perimeter.
00:53 Next, we'll find the perimeter of the square.
00:57 We have the length of one side of the square.
01:01 The square's perimeter is 4 times one side length.
01:08 Let's insert the side value and work out the square's perimeter.
01:21 Here is the square's perimeter.
01:28 And that's how we solve which shape has the smaller perimeter.

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

The radius of a circle is 3 centimeters.

The length of the side of a square is 7 centimeters.

Which shape has a larger perimeter/circumference?

2

Step-by-step solution

To solve the problem of determining which shape has a larger perimeter or circumference, let's apply the necessary formulas:

The given information is as follows:

  • Radius of the circle: r=3cm r = 3 \, \text{cm}
  • Side length of the square: s=7cm s = 7 \, \text{cm}

Step 1: Calculate the circumference of the circle

The formula for the circumference of a circle is C=2πr C = 2 \pi r . Thus, substituting the given radius, we get:

C=2π(3)=6π C = 2 \pi (3) = 6 \pi

Using an approximation for π3.14159\pi \approx 3.14159, the circumference is:

C6×3.1415918.84954cm C \approx 6 \times 3.14159 \approx 18.84954 \, \text{cm}

Step 2: Calculate the perimeter of the square

The formula for the perimeter of a square is P=4s P = 4s . Substituting the given side length, we find:

P=4×7=28cm P = 4 \times 7 = 28 \, \text{cm}

Step 3: Compare the perimeter of the square and the circumference of the circle

The calculated circumference of the circle is approximately 18.85cm 18.85 \, \text{cm} , and the perimeter of the square is 28cm 28 \, \text{cm} .

Upon comparison, 28cm 28 \, \text{cm} (perimeter of the square) is greater than 18.85cm 18.85 \, \text{cm} (circumference of the circle).

Therefore, the shape with the larger perimeter/circumference is the square.

3

Final Answer

The circle

Key Points to Remember

Essential concepts to master this topic
  • Formulas: Circle circumference C=2πr C = 2\pi r , square perimeter P=4s P = 4s
  • Calculation: Circle: 2π(3)=6π18.85 2\pi(3) = 6\pi \approx 18.85 cm vs Square: 4(7)=28 4(7) = 28 cm
  • Check: Compare final numerical values: 18.85 cm < 28 cm ✓

Common Mistakes

Avoid these frequent errors
  • Comparing exact and approximate values incorrectly
    Don't compare 6π 6\pi directly to 28 without calculating! This leads to confusion about which is larger. Always convert 6π 6\pi to its decimal approximation (18.85) before comparing with 28.

Practice Quiz

Test your knowledge with interactive questions

\( r=11 \)

Calculate the circumference.

111111

FAQ

Everything you need to know about this question

Do I need to memorize the exact value of π?

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No! You just need to know that π ≈ 3.14159 for calculations. Most problems accept π3.14 \pi \approx 3.14 as sufficient for comparisons.

Why is the answer 'the square' when the explanation shows the circle has the larger perimeter?

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There's an error in the original explanation! The calculation is correct: circle = 18.85 cm, square = 28 cm. Since 28 > 18.85, the square actually has the larger perimeter, not the circle.

Can I leave my answer as 6π instead of calculating the decimal?

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For comparison problems, it's better to convert to decimals. While 6π 6\pi is exact, converting to 18.85 makes it much easier to compare with 28.

What if the question asked for the exact difference?

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Then you'd calculate: 286π 28 - 6\pi for the exact answer, or 2818.85=9.15 28 - 18.85 = 9.15 cm for the approximate difference.

How do I remember which formula goes with which shape?

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Think about the shape: A circle goes around (circumference = 2πr 2\pi r ), while a square has 4 equal sides (perimeter = 4s 4s ).

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