21Acirclesliteracy

toc = Circle Geometry =

Parts of a circle

 * You should already be familiar with the following terms: **


 * Centre **
 * The middle point of the circle.[[image:bhs-methods10/09Acirc1.gif align="right"]]
 * Each point on the circumference is an equal distance from the centre.


 * Circumference **
 * The outside boundary of the circle.
 * The perimeter of the circle


 * [[image:bhs-methods10/09Acirc2.gif align="right"]]Radius **
 * A straight line from the centre to the circumference of the circle (in any direction).
 * The radius is half the length of the diameter


 * Diameter **
 * A straight line all the way from one side of the circle to the other, passing through the centre.
 * The diameter is twice the length of the radius.


 * The following terms may be new to you: **
 * Chord **
 * A straight line between any two points on the circumference
 * The diameter is one example of a chord but chords don't have to go through the centre.


 * Segment **
 * The __area__ between a chord and the circumference
 * A ** minor segment ** is the smaller of the two possible segments based on a chord
 * A ** major segment ** is the larger of the two possible segments based on a chord


 * Sector **
 * The area enclosed by two radii and the circumference (like a slice of pizza)
 * The angle at the centre between the two radii determines the size of the sector
 * A ** minor sector ** is the smaller of the two possible sectors based on two radii
 * A ** major sector ** is the larger of the two possible sectors based on two radii


 * [[image:bhs-methods10/09Acirc3a.gif width="135" height="317" align="right"]]Arc **
 * A portion of the circumference
 * The curved line between any two points on the circumference
 * The angle at the centre between the two endpoints of the arc determines the size of the arc


 * Tangent **
 * A straight line outside the circle that touches the circle at exactly one point
 * The word __**tangent**__ is used in this way for a straight line touching any type of curve at one point.


 * Secant **
 * A straight line that extends a chord beyond the edge of the circle in one direction
 * The __**secant**__ is used in some aspects of trigonometry


 * Subtend **
 * An arc is said to __**subtend**__ an angle where the two lines that create the angle make the endpoints of the arc.
 * Most commonly, the vertex of the angle subtended by the arc is either at the centre of the circle or at a point on the opposite circumference of the circle.
 * In both circles below, the arc ** AC __subtends__** the angle ** Ð **** ABC **.
 * Or, the angle ** Ð **** ABC ** is **__subtended__** by the arc ** AC **.

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