Radius facts for kids
In geometry, the radius is a special line that helps us understand the size of circles and spheres. Imagine a line starting exactly from the center of a circle or sphere and going straight out to its edge. That line is the radius! We often use the letter r to stand for radius in math.
The radius is the shortest way to connect the center to the outside edge. For a circle, it goes from the center to the perimeter (the line that forms the circle). For a sphere, it goes from the center to its surface (the outside skin of the ball).
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Radius and Diameter
The radius is directly connected to another important measurement called the diameter. The diameter is a straight line that goes all the way across a circle or sphere, passing through its center.
Think of it this way:
- The diameter is like two radii (plural of radius) put together, end-to-end.
- So, the radius is always exactly half of the diameter.
Here are the simple math rules for them:
- If r is the radius and d is the diameter:
* r = d ÷ 2 * d = 2 x r (or d = r + r)
Radius, Circumference, and Area
The radius is super important because it helps us figure out other cool things about circles, like their circumference and area.
- The circumference is the distance all the way around the circle, like walking along its edge.
* The relationship between the radius 
and the circumference 
of a circle is: 
* Here, 
(pronounced "pi") is a special number, about 3.14159.
- The area is the amount of flat space inside the circle.
* The area 
of a circle of radius is: 
* The 
part means "r multiplied by r" (r x r).
Why is the Radius Important?
The radius is a basic building block in geometry. It's used in many real-world situations:
- When you buy a pizza, its size is often described by its diameter, which means you can easily find its radius.
- Engineers use the radius to design wheels, gears, and pipes.
- Scientists use it to calculate the size of planets or the range of a radio signal.
Related Pages
Images for kids
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A square, for example (n=4)
More to Explore
In Spanish: Radio (geometría) para niños
