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Radius, Diameter, Circumference And Area Of Circles

Grade 7 Math Worksheets

Welcome to the fascinating world of circles in mathematics! Circles are fundamental shapes that appear in countless aspects of our lives, from the wheels of a car to the planets in our solar system. In this guide, we’ll explore some key concepts related to circles: radius, diameter, circumference, and area.

Radius (r):

The radius of a circle is a line segment that connects the center of the circle to any point on its circumference. In simpler terms, it’s the distance from the center of the circle to any point on its edge. If you were to draw a line from the center of the circle to its edge, you’d have the radius.

Diameter (d):

The diameter of a circle is the longest distance that can be drawn across the circle, passing through its center and touching two points on the circumference. It’s essentially twice the length of the radius. So, if the radius is like the “half-width” of the circle, the diameter is the full width.

Relationship between Radius and Diameter:

Diameter (d) = 2 × Radius (r)

Radius (r) = 1/2 × Diameter (d)

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Circumference (C):

The circumference of a circle is the distance around its outer edge. Imagine you’re walking along the edge of a circular track; the distance you cover is the circumference. There’s a special relationship between the circumference, diameter, and a mathematical constant called π (pi). This relationship is given by the formula:

Circumference (C) = π × Diameter (d)

or

Circumference (C) = 2 × π × Radius (r)

Here, π is a constant approximately equal to 3.14159. It’s an irrational number, which means its decimal representation goes on forever without repeating.

Area (A):

The area of a circle is the measure of the region enclosed by the circle’s circumference. It’s the space inside the circle. The formula for finding the area of a circle is:

Area (A) = π × Radius (r)²

or

Area (A) = π × (Diameter (d) / 2)²

Key Points to Remember:

• Radius (r) is the distance from the center of the circle to any point on its edge.
• Diameter (d) is the longest distance across the circle, passing through its center.
• Circumference (C) is the distance around the outer edge of the circle.
• Area (A) is the measure of the space enclosed by the circle’s circumference.

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Solved Examples

Let’s explore some examples to illustrate the concepts of radius, diameter, circumference, and area of circles:

Example 1: Calculating Circumference and Area

Suppose we have a circular garden with a radius of 5 meters.

1. Circumference (C):

Using the formula: Circumference (C) = 2 × π × Radius (r)
C = 2 × 3.14159 × 5
C ≈ 31.4159 meters

2. Area (A):

Using the formula: Area (A) = π × Radius (r)²
A = 3.14159 × 5²
A ≈ 78.5398 square meters

Example 2: Finding Diameter

Imagine we have a circular table with a diameter of 36 inches.

1. Radius (r):

Using the relationship: Radius (r) = 1/2 × Diameter (d)

r = 1/2 × 36

r = 18 inches

Example 3: Comparing Circumference and Area

Let’s compare the circumference and area of a pizza with a diameter of 12 inches.

1. Circumference (C):

Using the formula: Circumference (C) = π × Diameter (d)
C = 3.14159 × 12
C ≈ 37.6991 inches

2. Area (A):

Using the formula: Area (A) = π × (Diameter (d) / 2)²
A = 3.14159 × (12 / 2)²
A ≈ 113.097 square inches

Example 4: Calculating Radius from Circumference

Suppose we have a circular pond with a circumference of 50 meters.

1. Radius (r):

Using the formula: Radius (r) = Circumference (C) / (2 × π)

r = 50 / (2 × 3.14159)

r ≈ 7.9577 meters

Radius, Diameter, Circumference And Area Of Circles FAQS

What is a circle?

A circle is a closed curve in which all points are equidistant from a fixed point called the center.

What is the radius of a circle?

The radius of a circle is the distance from the center of the circle to any point on its circumference. It’s represented by the symbol “r.”

What is the diameter of a circle?

The diameter of a circle is the longest distance that can be drawn across the circle, passing through its center and touching two points on the circumference. It’s represented by the symbol “d.”

How is the radius related to the diameter?

The diameter of a circle is twice the length of its radius. In other words, Diameter (d) = 2 × Radius (r).

What is the circumference of a circle?

The circumference of a circle is the distance around its outer edge. It’s calculated using the formula: Circumference (C) = π × Diameter (d) or Circumference (C) = 2 × π × Radius (r).

What is π (pi)?

π (pi) is a mathematical constant representing the ratio of the circumference of a circle to its diameter. It’s approximately equal to 3.14159 and is an irrational number, meaning its decimal representation goes on infinitely without repeating.

How do you calculate the area of a circle?

The area of a circle is the measure of the region enclosed by its circumference. It’s calculated using the formula: Area (A) = π × Radius (r)².

What units are used to measure the radius, diameter, circumference, and area of circles?

These measurements can be in any appropriate unit of length, such as meters, centimeters, inches, or feet, depending on the context of the problem.

Gloria Mathew writes on math topics for K-12. A trained writer and communicator, she makes math accessible and understandable to students at all levels. Her ability to explain complex math concepts with easy to understand examples helps students master math. LinkedIn

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