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CirclesGeometryMiddle schoolabout 1:00

A circle has radius 9. Find its radius, diameter, circumference and area: animated lesson

A narrated, step-by-step circles video lesson with captions. Final answer: r = 9, d = 18, C = 18π ≈ 56.54866776, A = 81π ≈ 254.4690049.

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Video with a synthetic English voice and captions. Pick another language for subtitles in that language, or open the interactive version.

Video narrated by a synthetic voice (Piper TTS, en_US-ljspeech-high voice (LJ Speech dataset, public domain)); the steps come from our maths engine and the answer was checked independently. Download MP4 (0.4 MB) · Captions (WebVTT)

Steps on screen

1. The problem
Worked example
A circle has radius 9. Find its radius, diameter, circumference and area.
2. Step by step
1. $d = 2r = 18$
  Diameter
2. $C = 2\pi r = 2\pi \cdot 9 = 18\pi \approx 56.54866776$
  Circumference
3. $A = \pi r^2 = \pi \cdot 9^2 = 81\pi \approx 254.4690049$
  Area
3. The answer
$r = 9$
r = 9, C = 18π ≈ 56.54866776, A = 81π ≈ 254.4690049
4. Check
- C/d = 3.141592654 = π.
- A = C·r/2 = 254.4690049.

Transcript

The problem. Here is the problem: A circle has radius 9. Find its radius, diameter, circumference and area. Find the radius, diameter, circumference and area of a circle from any one of them.

Step by step. Diameter. Circumference. Area.

The answer. So the answer is r equals 9, d equals 18, C equals 18pi approximately 56.54866776, A equals 81pi approximately 254.4690049. Every measurement of a circle follows from its radius. The circumference grows in proportion to the radius and the area in proportion to its square.

Check. C/d equals 3.141592654 equals pi. A equals C times r/2 equals 254.4690049.

About circles

Radius, diameter, circumference and area of a circle.

Circles: A circle has radius 9. Find its radius, diameter…