Definitions

A circle is a two-dimensional pattern that consists of all the points that are within a certain distance from a given central point. The circle is named after its central point.

Circle - related definitions

  • The perimeter of a circle is a plane curve that consists of all the points that have the same distance from the circle's centre. 
  • The radius of a circle is a segment that connects the centre of the circle with a point at its perimeter. 

  • A chord is a segment that connects two points on the circumference of a circle. 

  • The diameter is the chord that passes through the circle's central point. 

  • A tangent is a line that flanks a circle at one point. 
  • Two points on the perimeter of a circle divide its circumference into two arcs. 




Parts of a circle O
  • radius OC
  • diameter AB or chord DE
  • chord DE
  • arc CB

Areas of a circle

  • A sector is an area of ​​a circle bounded by two radii. 
  • A sector's central angle [[$\alpha$]] has its vertex at the circle's centre.  
  • A chord divides the circle into two segments. 

 

Since each chord divides a circle into two segments, it is important to clearly mark which segment is meant. The same applies to sectors.

The diameter of a circle

The diameter [[$ d $]] of a circle is twice the length of its radius [[$ r $]].
[[$d = 2r$]]