Look at the circle in the figure:
Its radius is equal to 4.
What is its circumference?
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Look at the circle in the figure:
Its radius is equal to 4.
What is its circumference?
The formula for the circumference is equal to:
8π
O is the center of the circle in the diagram.
What is its perimeter?
The circumference formula C = 2πr comes from the relationship between radius and diameter. Since diameter = 2 × radius, and circumference = π × diameter, we get C = π(2r) = 2πr.
Unless specifically asked for a decimal approximation, leave your answer as 8π. This is the exact value and is preferred in mathematics over the approximate decimal 25.13.
Radius is the distance from center to edge (4 in this problem). Diameter is the distance across the entire circle through the center, which equals 2 × radius = 8.
Think: "Circle goes around 2-pi-R times" where R is radius. Or remember that circumference = π × diameter, and diameter = 2 × radius, so C = π(2r) = 2πr.
If given diameter, use C = πd directly, or divide diameter by 2 to get radius, then use C = 2πr. Both methods give the same answer!
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