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## Class 9 (Foundation)

### Course: Class 9 (Foundation)>Unit 11

Lesson 1: Circumference of a circle

# Radius & diameter from circumference

Sal finds the radius and diameter of a circle given the circumference.

## Want to join the conversation?

• I need help, How do u find the circumference with the given radius?
• The circumference given the radius can be found using C = 2πr
C is the circumference
• um i really need help with this i keep getting confused with circumference and radius
• The circumference is the distance around the circle. In other words, the circle's perimeter.

The diameter is a straight line that passes through the center of the circle.

The radius is half of the diameter. It starts from a point on the circle, and ends at the center of the circle.

Hope this helps!
• no it did not help what is 3.14
• 3.14 is π (pi, and infinite number) rounded to the nearest hundredth.
• who else is studying in their summer break cause i am

asian parents
• fr bro im in 3rd grade and this topic kind of hard but can get this!
• okay so if to find circumference= 2pir could it just be diameter x pi? To find circumference?
• Yes, both ways work. The only reason to use one or the other is if you are only given the number of the radius or if you are only given the number of the diameter.
• do you guys have a step by step for the area of the circle because i am not understanding because i am in 7th grade and i need to know more
• Imagine you have a round pizza. The area of a circle is like the amount of space inside that pizza. It tells you how much surface area the circle covers.

To find the area of a circle, we use a special number called pi, which is approximately 3.14. Pi helps us figure out how much space is inside the circle.

The formula to find the area of a circle is: Area = pi times the radius squared.

The radius is a line that starts at the center of the circle and goes to the edge. If you cut the circle in half, the radius would be the distance from the center to the edge.

To find the area, we need to know the radius. Once we have the radius, we square it by multiplying it by itself. Then, we multiply that squared radius by pi.

For example, let's say the radius of a circle is 5 units. We square the radius: 5 x 5 = 25. Then, we multiply the squared radius by pi: 25 x 3.14 = 78.5.

So, the area of the circle with a radius of 5 units is 78.5 square units. It means that if you measured the inside of that circle, it would cover an area of 78.5 square units.

Remember, the area is a measure of space inside the circle, like the space on a pizza. And we find it using the formula: Area = pi times the radius squared. please upvote if this helps
• so the diameter x Pi = my answer? i feel a tad bit lost.
• yes C=πd or C=2πr which is radius times 2 times pi
• A wet bicycle tire leaves a trace of water on the floor. The tire has a radius of 30, and the bicycle wheel makes 3 full rotations before stopping.
How long is the trace of water left on the floor?

I don't get that
• Each rotation produces a trace of water with length equal to the wheel’s circumference. Since the radius is 30, the circumference is 2pi*30 = 60pi.

So 3 rotations would produce a trace of water with length 3*60pi = 180pi.
• So circumference and diameter are always the same numbers?
• The circumference is the diameter times pi (𝝅).
`Example:`
``If the diameter is 12 then the circumference is 12𝝅``

𝝅 is generally difficult to calculate, since it goes on forever you can write it in terms of 𝝅 (12𝝅).

You could also times the diameter by 3.14 to estimate it.
`Example:`
``If the diameter is 12 then the circumference is 37.68.``

`Hope This Helps!`