# Radius of a Circle – Definition, Formula, Facts, Examples, FAQs

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## What Is the Radius of a Circle?

The radius of a circle represents the distance between the center and the boundary of the circle (circumference). The radius is generally denoted by “R” or “r.”

The word “radius” is the singular form, while “radii” is the plural form.

Note that 3D shapes such as spheres, cylinders, cones, shapes with circular bases also have a radius.

## Radius of a Circle: Definition

The measure of distance between the center of a circle and any point on the boundary of the circle is called its radius.

## Radius of a Circle: Formulas

We can express radius in terms of diameter, circumference, or area of a circle. Before we go into that, let’s quickly look into what these terms represent.

Diameter is the length of a line segment passing through the center of the circle and connecting any two points on the circumference (boundary).  Half of the diameter is the radius.

Diameter $= 2 \times Radius$

Radius $= \frac{Diameter}{2}$

r $= \frac{d}{2}$

Circumference is the total distance of the boundary of the circle given by 2r.

Circumference $= C = 2\pi r$

$r = \frac{C}{2\pi}$

The space or region enclosed within the circumference of the circle is its area.

Area of a circle $= \pi r^{2}$

Radius = $\sqrt{\frac{Area}{\pi}}$

## Equation of a Circle with Radius r

The equation of a circle having radius r and center at origin (0, 0) is given by

$x^{2} + y^{2} = r^{2}$

Equation of a circle having radius r and with center (h, k) is given by

$(x\;-\;h)^{2} + (y\;-\;k)^{2} = r^{2}$

• A circle with a radius 1 unit is called a unit circle.
• A circle is a flat two-dimensional shape made up of a collection of points, all of which lie at the same distance from its center. This distance is called radius.
• The radius of a circle and tangent are perpendicular at the point of contact.
• The area and circumference are directly proportional to the radius.

## Conclusion

A circle is a very common shape found numerous times in our day-to-day life. Beginning from glasses and sunlight, to ending the day with the moon and clock, there are multiple circles. Thus, knowing the formula for its measurement is necessary. The radius of the circle is the distance from the center to the circumference. It is related to the diameter and area of the circle as well.

## Solved Examples on Radius of Circle

1. A wire measures 10 units in length. Find the radius of a circle formed by bending the wire and joining its two ends? (Use $\pi = 3.14$).

Solution:

A circle is made by bending the wire of length 10 units. Thus, the circumference of the circle is 10 units.

C $= 2\pi r$

$10 = 2r\pi$

Radius $= r = \frac{10}{2\pi}$

Radius $= r = \frac{10}{2\pi}$

Radius $= r = 1.6$ units

2. What is the radius of the given circle?

Solution:

The given line segment is the diameter of the circle.

Diameter = 25 in

Radius $= \frac{Diameter}{2}$

Radius $= \frac{25}{2}$

3. A circular garden spans a region of 50.24 square feet. Find the radius.

Solution: The area of a circle is given by

Area $= \pi r^{2}$

Radius  $= \sqrt{\frac{Area}{\pi}}$

Radius $= \sqrt{\frac{50.24}{3.14}}$

Radius $= \sqrt{\frac{50.24}{3.14}}$

Radius $= \sqrt{16}$

5. What is the radius of a circle with its center at origin, if (2, 4) is a point on the circumference of a circle?

Solution:

The equation for the radius of the circle with radius r and center (0, 0) is $x^{2} + y^{2} = r^{2}$.

The (2, 4) point lies on the circle, so it satisfies the equation of the circle.

$2^{2} + 4^{2} = r^{2}$

$r^{2} = 4 + 16$

$r^{2} = 20$

$r = \sqrt{20}$

r = 4.5 units

## Practice Problems on Radius of Circle

1

### What is the radius if the circumference of a circle is 20 units?

1.8
3.2
6
10
CorrectIncorrect
Radius $= \frac{Circumference}{2\pi}$
2

### What is the meaning of the radius of a circle?

Distance from the center to the circumference
Distance from one end of the circle to the other end
Distance between any two points on the boundary
CorrectIncorrect
Correct answer is: Distance from the center to the circumference
Radius is the distance between the center and the boundary of the circle.
3

### How to find the radius from the diameter?

By dividing it by 2
By multiplying it by 2
By adding 2 to the diameter
By subtracting 2 from the diameter
CorrectIncorrect
Correct answer is: By dividing it by 2
The radius is half of the diameter.
4

### If the radius increases, the area and circumference will

increase.
decrease.
stay the same.
double.
CorrectIncorrect
If the radius increases, the area and circumference of the circle will also increase. It is because both area and circumference are directly proportional to the radius.
5

### What is the diameter of a unit circle?

1 unit
0.5 unit
2 units
4 units
CorrectIncorrect
Radius of unit circle = 1 unit
Diameter of unit circle = 2r = 2 units