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Section 8.2 · Geometry and measure

The parts of a circle

Name the parts of a circle and use the relationship between radius and diameter.

6 teaching slides 47 practice questions Worked answers for every question

1 · Teaching slidesuse ← → keys

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2 · Key words

centreThe exact middle of the circle.
radiusThe distance from the centre to the edge.
diameterThe distance right across, through the centre; twice the radius.
circumferenceThe distance all the way round the outside.
chordA straight line joining two points on the circle.
arcPart of the circumference.

3 · Summary of the sectionrevise from this

  • The centre is the middle; the radius reaches from it to the edge.
  • The diameter goes right across through the centre, so d = 2r and r = d ÷ 2.
  • The circumference is the distance all the way round — the perimeter of a circle.
  • A chord joins two points on the circle. The diameter is the longest chord.
  • An arc is a part of the circumference.
  • A circle has infinitely many lines of symmetry — every line through the centre.
  • Its order of rotational symmetry is infinite: any turn about the centre leaves it unchanged.
  • Read the question carefully: “how wide across” means the diameter, “how far from the middle” means the radius.
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Video lessons1 videos

Math Antics - Circles, What Is PI?

American mathantics

The parts of a circle, and the link between diameter and circumference.

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4 · Practice47 questions, easiest first

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8.1 Polygons
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8.3 Symmetry