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## Class 7 math (India)

### Course: Class 7 math (India)>Unit 9

Lesson 4: Circles

# 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.
• 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.
• Chicken sandwich
• 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
• Do you ever feel like a plastic bag, storing through the wind, wanting to start again?
• hi this help a lot
• 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.
• 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