Chord
From Greek: khorde "gut, string."
A line that links two points on a circle or curve. (pronounced "cord")
Try this Drag either orange dot. The blue line will always remain a chord to the circle.
The blue line in the figure above is called a "chord of the circle c". A chord is a lot like a
secant, but where the secant is a line stretching
to infinity in both directions, a chord is a
line segment that only covers the part inside the circle.
A chord that passes through the center of the circle is also a diameter of the circle.
Calculating the length of a chord
Two formulae are given below for the length of the chord,. Choose one based on what you are given to start.
1. Given the radius and central angle
Below is a formula for the length of a chord if you know the radius and central angle.
where
r is the radius of the circle
c is the angle subtended at the center by the chord
sin is the sine function (see Trigonometry Overview)
2. Given the radius and distance to center
Below is a formula for the length of a chord if you know the radius and the perpendicular distance from the chord to the circle center.
This is a simple application of
Pythagoras' Theorem.
where
r is the radius of the circle
d is the perpendicular distance from the chord to the circle center
Finding the center
The perpendicular bisector of a chord always passes through the center of the circle. In the figure at the top of the page, click "Show Right Bisector". Then move
one of the points P,Q around and see that this is always so. This can be used to find the center of a circle: draw one chord and its right bisector.
The center must be somewhere along this line.
Repeat this and the two bisectors will meet at the center of the circle.
See Finding the Center of a Circle in the Constructions chapter for stepbystep instructions.
Intersecting Chords
If two chords of a circle intersect, the intersection creates four line segments that have an interesting relationship. See Intersecting Chord Theorem.
Other circle topics
General
Equations of a circle
Angles in a circle
Arcs
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