circumference: The distance around a circle.

pi: The ratio of circumference to diameter. The ratio is the same for every circle and is represented by the Greek letter π , called pi. Pi is approximately equal to 3.14 or ²²/₇.

The ratio of the circumference of a circle to the diameter is π. Use algebra and this ratio to write a formula for the circumference of a circle.

Step 1: Write the ratio as an equation. Use C for circumference and d for diameter.

Step 2: Multiply each side of the equation by the diameter, d.

Step 3: Write the equation so that circumference is on the left side of the equal sign.

C = πd

Formulas

Circumference = πd or 2πr, where d is diameter and r is radius.

Area = πr²

Area

Find the area of the circle to the nearest tenth. Use 3.14 for π.

Step 1: Use the formula.

A = πr²

Step 2: Replace with the values from the problem. The problem tells you to use the decimal approximation for π, 3.14. When you replace π with 3.14, use ≈ instead of the equal sign. The sign ≈ means approximately.

A ≈ (3.14)3²

Step 3: Do the computations. Evaluate the power first. Then multiply.

A ≈ (3.14)3²
A ≈ (3.14)9
A ≈ 28.26

Step 4: Write the answer in a complete sentence. Round the area to the nearest tenth. Remember to include the units.

The area of the circle is about 28.3 m².

Test-Taking Hint

Area problems for circles will often tell you what form of pi to use. Read carefully!

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Circle Measurements