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A circle with a number of its mathematical parts labelled. And someone trying to climb up the tangent.

Parts of a circle

A recap of some of the fundamental parts of a circle, including some that are easy to forget:

  • Diameter — a line crossing the circle through its centre. Also the longest straight line you can draw in a circle.
  • Radius — a line from the centre to any point on the edge. Also, half the diameter.
  • Sector — the classic Trivial Pursuit cheese, pizza or cake slice formed by two radii.
  • Area — equal to pi x the radius squared.
  • Circumference — equal to pi x the diameter.
  • Arc — a section of the circumference.
  • Chord — a straight line touching any two points of a circle. The longest chord is the diameter.
  • Segment — the areas on either side of a chord, major and minor.
  • Tangent — a straight line just touching the edge of the circle without crossing it. It forms a right angle to the radius on the edge. Just as if you were able to lean a ladder against one.

More maths and science refreshers: States of matter, BODMAS (or PEMDAS), the 9 times table on your fingers, binary, incidence and reflection

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