Concept 7 of 10Foundation
Watch on YouTubeVideoCircles — circumference and area
A circle is a perfectly round shape. Its key parts:
- Centre: fixed middle point
- Radius (r): centre to edge
- Diameter (d): edge to edge through centre; d = 2r
- Circumference (C): distance around the circle (perimeter)
- Chord: any segment joining two points on the circle
- Arc: a portion of the circumference
Two great formulas, both involving π (pi):
- C = 2πr = πd
- A = πr²
π is a universal constant — the SAME ratio (circumference ÷ diameter) for every circle ever. π ≈ 3.14, or 22/7 when numbers divide cleanly.
Example
Bicycle wheel, r = 35 cm (π = 22/7):
• Circumference = 2 × 22/7 × 35 = 220 cm (distance rolled in 1 turn)
• Area = 22/7 × 35 × 35 = 3,850 cm²
Wheel rolls 1 km. How many turns? 100,000 ÷ 220 ≈ 454 turns.
• Circumference = 2 × 22/7 × 35 = 220 cm (distance rolled in 1 turn)
• Area = 22/7 × 35 × 35 = 3,850 cm²
Wheel rolls 1 km. How many turns? 100,000 ÷ 220 ≈ 454 turns.
💡 Tip:π is roughly 3 for quick estimates. Use 22/7 when radius is 7, 14, 21, 28, 35… Use 3.14 otherwise.
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5 questions to check what you just read.
0 / 5
- Q1.Circumference of circle radius 7 (π = 22/7):
- Q2.Area of circle radius 7 (π = 22/7):
- Q3.Diameter if radius = 14:
- Q4.If radius doubles, area becomes:
- Q5.Circumference 44 cm; radius (π=22/7):