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PEAZIT Aha card

Circumference is about three times the diameter

Circumference divided by diameter is always π. Thus C = πd; the diameter is twice the radius.

Using π ≈ 3.14, radius 5 cm gives circumference 31.4 cm.

Relation	Meaning
d = 2r	A diameter crosses the center and contains two radii.
o = πd = 2πr	Multiply the diameter by π, or the radius by 2π.
r = 5; o ≈ 31.4 cm	2 × 3.14 × 5 = 31.4.

One worked example

d = 2rA diameter crosses the center and contains two radii.
o = πd = 2πrMultiply the diameter by π, or the radius by 2π.
r = 5; o ≈ 31.4 cm2 × 3.14 × 5 = 31.4.

Watch out: Circumference uses length units, not square units. 3.14 is only an approximation to π.

Goal, prerequisites and card origin

What you will learn: Find circumference from a radius or diameter.

What you need first: Multiplication and circle radius.

An original PEAZIT working card. Independent expert review and effectiveness testing have not yet taken place. · v1

Show connections

Circumference divided by diameter is always π. Thus C = πd; the diameter is twice the radius.

Without JavaScript, write your answer elsewhere before opening the solution.The radius is 4 cm. Use π = 3.14. What is the circumference in cm? Enter a number.

Solution — open after your own attempt

25.12 2 × 3.14 × 4 = 25.12 cm.

A question for your next return: The diameter is 6 cm. Using π = 3.14, find the circumference in cm. Enter a number.

Solution — open after your own attempt

18.84 3.14 × 6 = 18.84 cm; do not double a given diameter.

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