Circumference - Using 3.14 For Pi

🔑 Key Concepts

• The circumference of a circle is the distance around the circle.
• You can find circumference using either the radius or the diameter.
• If you know the radius, use the formula \( C = 2\pi r \).
• If you know the diameter, use the formula \( C = \pi d \).
• When using 3.14 for \( \pi \), the answer should be written as approximately equal to.

✏️ Worked Examples — Finding the Circumference of a Circle

Example 1 — Radius Given

Find the circumference using \(3.14\) for \( \pi \).

We are given the radius, so use:

\( C = 2\pi r \)

\( C \approx 2(3.14)(4.5) \)

\( C \approx 6.28(4.5) \)

\( C \approx 28.26 \)

\( C \approx 28.26 \text{ cm} \)

Example 2 — Diameter Given

Find the circumference using \(3.14\) for \( \pi \).

We are given the diameter, so use:

\( C = \pi d \)

\( C \approx 3.14(12) \)

\( C \approx 37.68 \)

\( C \approx 37.68 \text{ cm} \)

Example 3 — Diameter Given

Find the circumference using \(3.14\) for \( \pi \).

We are given the diameter, so use:

\( C = \pi d \)

\( C \approx 3.14(8) \)

\( C \approx 25.12 \)

\( C \approx 25.12 \text{ cm} \)

🧠 Math Vocabulary

• Circumference: The distance around a circle.
• Radius: The distance from the center of a circle to the edge.
• Diameter: The distance across a circle through the center.
• Pi (π): A number used to calculate circumference and area of circles.
• Approximate: A value that is close to the exact answer.

💡 Main Idea

To find the circumference of a circle, identify whether the radius or diameter is given and then choose the correct formula. When using 3.14 for \( \pi \), the result is an approximation, so the answer should include the approximately equal to symbol.

📚 What You Should Already Know

Students should understand multiplication with decimals and know the difference between radius and diameter before solving circumference problems.

🚀 What Comes Next

After learning how to find circumference, students will use similar formulas to find the area of a circle and solve real-world problems involving circular objects.

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