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CirclesGeometryMiddle schoolabout 1:03

A circle has diameter 15. Find its radius, diameter, circumference and area: animated lesson

A narrated, step-by-step circles video lesson with captions. Final answer: r = 15/2, d = 15, C = 15π ≈ 47.1238898, A = 225π/4 ≈ 176.7145868.

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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 diameter 15. Find its radius, diameter, circumference and area.
2. Step by step
1. $r = \frac{d}{2} = \frac{15}{2}$
  Radius from the diameter
2. $C = 2\pi r = 2\pi \cdot \frac{15}{2} = 15\pi \approx 47.1238898$
  Circumference
3. $A = \pi r^2 = \pi \cdot \frac{15}{2}^2 = \frac{225\pi}{4} \approx 176.7145868$
  Area
3. The answer
$r = \frac{15}{2}$
r = 15/2, C = 15π ≈ 47.1238898, A = 225π/4 ≈ 176.7145868
4. Check
- C/d = 3.141592654 = π.
- A = C·r/2 = 176.7145868.

Transcript

The problem. Here is the problem: A circle has diameter 15. 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. Radius from the diameter. Circumference. Area.

The answer. So the answer is r equals 15 over 2, d equals 15, C equals 15pi approximately 47.1238898, A equals 225pi/4 approximately 176.7145868. 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 176.7145868.

About circles

Radius, diameter, circumference and area of a circle.

A circle has diameter 15. Find its radius, diameter…