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Circles: circumference and area
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Every circle shares a special number, π (pi), which is about 3.14. It links the diameter to the distance around the edge.
Parts of a circle
Radius: centre to edge. Diameter: right across through the centre (2 × radius).
Circumference: the distance around the edge.
Chord: a straight line joining two points on the edge. Tangent: a straight line that touches the circle at one point.
Arc: part of the circumference. Sector: a slice, like a pizza slice. Segment: the area between a chord and an arc.
Circumference: the distance around the edge.
Chord: a straight line joining two points on the edge. Tangent: a straight line that touches the circle at one point.
Arc: part of the circumference. Sector: a slice, like a pizza slice. Segment: the area between a chord and an arc.
Circumference
C = πd (or C = 2πr).
Diameter 10 cm: C = π × 10 = 31.4 cm (1 d.p.).
Diameter 10 cm: C = π × 10 = 31.4 cm (1 d.p.).
Area
A = πr2. Square the radius, not the diameter.
Radius 6 cm: A = π × 36 = 113.1 cm2 (1 d.p.).
In terms of π: radius 5 cm gives A = 25π cm2.
Radius 6 cm: A = π × 36 = 113.1 cm2 (1 d.p.).
In terms of π: radius 5 cm gives A = 25π cm2.
Find the perimeter of a semicircle with diameter 10 cm, to 1 d.p.
- Curved edge: half of π × 10 = 5π = 15.71...
- Add the straight edge (the diameter): 15.71 + 10
Answer: 25.7 cm
C = πd and A = πr2. Use the π button on your calculator, and halve the diameter to get the radius before finding an area.
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