A circle is a closed figure defined by a set of points equidistant from a fixed point called the centre. The distance between the centre and any point on the circle is called the radius, and the distance across the circle through the centre is called the diameter.

In geometrical language, a circle is defined as a set of points in a plane at equal distances from a fixed point called the centre. For example, imagine a Ferris wheel with people seated in each compartment. As the wheel rotates, each person moves in a circular path that is equidistant from the centre of the wheel. The distance from the centre of the Ferris wheel to any of its compartments is the circle's radius, and the distance around the circle is its circumference.

Suppose the Ferris wheel has a radius of 10 meters. To find its circumference, we can use the formula C = 2πr, where r is the radius of the circle and π is a constant approximately equal to 3.14159. Plugging in the values, we get:

C = 2π(10) = 20π ≈ 62.83 meters

Therefore, the circumference of the Ferris wheel is approximately 62.83 meters.

So, a circle is a set of points equidistant from a fixed point called the centre. The distance from the centre to any point on the circle is the radius, and the distance around the circle is its circumference. We can use the formulas C = 2πr and A = πr2 to calculate the circumference and area of a circle, respectively.

## Calculating the circumference

The circumference of a circle is the distance around the circle, and the formula gives it:

C = 2πr

where C is the circumference, r is the radius, and π (pi) is a constant approximately equal to 3.14159.

The area of a circle is the amount of space inside the circle, and the formula gives it:

A = πr2

where A is the area and r is the radius.

Here are some examples of how to use these formulas:

1️⃣
Find the circumference and area of a circle with a radius of 5 cm.

C = 2πr = 2π(5) = 10π ≈ 31.42 cm
A = πr2 = π(52) = 25π ≈ 78.54 cm2

Therefore, the circle's circumference is approximately 31.42 cm, and the area of the circle is approximately 78.54 cm2.

2️⃣
Find the radius of a circle with a circumference of 25π cm.

C = 2πr
25π = 2πr
r = 25π / 2π
r = 12.5 cm

Therefore, the radius of the circle is 12.5 cm.

3️⃣
Find the diameter of a circle with an area of 36π cm2.

A = πr2
36π = πr2
r2 = 36
r = 6 cm

The radius of the circle is 6 cm, so the diameter is twice the radius:

d = 2r = 2(6) = 12 cm

Therefore, the diameter of the circle is 12 cm.

In summary, to find the circumference of a circle, we use the formula C = 2πr, where r is the radius. To find the area of a circle, we use the formula A = πr^2^. These formulas can be used to solve problems involving circles, such as finding the circumference, area, radius, or diameter of a given circle.

## Eating the Pi

Circles have unique characteristics that are essential to understand their relationships. These characteristics have specific terms associated with them that are important to recognise. One of the most important terms associated with circles is =="pi" - a mathematical constant that represents the ratio of the circumference of a circle to its diameter. Pi is an irrational number that has an infinite number of decimal places.

To calculate pi, one can measure the diameter (d) and the circumference (C) of several circular objects and divide C by d. The result of this calculation should consistently be a number slightly over 3. This number is the approximation of pi.

In any circle, pi is the circumference-to-diameter ratio. It is a constant with many applications in mathematics, including geometry, trigonometry, calculus, and physics.

In summary, Pi is a mathematical constant that represents the ratio of the circumference of a circle to its diameter. By measuring the diameter and circumference of a circle, we can calculate pi as the ratio of the two values. Pi is an essential constant in mathematics and has various applications across many disciplines.

## Circles Revision Quiz

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