Measures of a Circle
The three key measures of a circle are the radius, diameter, and circumference. In Grade 6 Saxon Math Course 1, students learn that the radius (r) is the distance from center to edge, the diameter (d) is the full distance across through the center, and d = 2r always. The circumference is the distance around the circle, approximated as C ≈ 3.14 × d. For a circular disk with diameter 10 cm: r = 5 cm and C ≈ 31.4 cm. Mastering these relationships is essential for all circle calculations.
Key Concepts
New Concept This lesson defines a circle's key parts. The diameter is the distance across a circle, the radius is from the center to the edge, and circumference is the distance around.
For any circle, the diameter is twice the length of the radius.
$$d = 2r$$ What’s next Next, you'll practice identifying these measures and use the relationship between the radius and diameter to solve problems.
Common Questions
What is the relationship between radius and diameter?
The diameter equals twice the radius: d = 2r. Equivalently, the radius is half the diameter: r = d/2.
A circle has a diameter of 10 cm. What is its radius?
r = 10 ÷ 2 = 5 cm.
What is the circumference formula using diameter?
C = π × d ≈ 3.14 × d.
Find the circumference of a circle with diameter 10 cm.
C ≈ 3.14 × 10 = 31.4 cm.
What is the difference between circumference and area?
Circumference is the distance around the edge (one-dimensional, measured in cm). Area is the space inside (two-dimensional, measured in cm²).