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What is a Chord?

A circle with several chords drawn inside it, with one chord highlighted and labeled
A chord is a line segment whose endpoints lie on the circle

In geometry, a chord is a straight line segment whose both endpoints lie on a circle. Think of it like a straight line that connects two points on a circle's edge.

The longest possible chord in any circle is called the diameter, which passes through the center of the circle. All other chords are shorter than the diameter.

What is a Perpendicular Bisector?

A line segment with a perpendicular line crossing exactly at its midpoint, forming perfect right angles
A perpendicular bisector crosses a line segment at its midpoint, forming 90-degree angles

A perpendicular bisector is a line that divides another line into two equal parts at a 90-degree angle. Let's break that down:

1

Bisector

It cuts something into two equal parts

2

Perpendicular

It forms a perfect right angle (90 degrees)

3

Together

It cuts a line in half at a perfect right angle

Every line segment has exactly one perpendicular bisector. This is a special property that makes perpendicular bisectors very important in geometry.

The Perpendicular Bisector of a Chord Theorem

A circle with a chord and its perpendicular bisector that clearly passes through the center of the circle
The perpendicular bisector of any chord always passes through the center of the circle

Now for the important theorem! The Perpendicular Bisector of a Chord Theorem states:

Theorem

The perpendicular bisector of any chord of a circle always passes through the center of the circle.

This means that if you draw a chord in a circle, and then draw a line that cuts that chord exactly in half at a right angle, that line will always go through the very center of the circle.

This is a special property that only circles have! No other shape has this property for all its chords.

Simple Proof of the Theorem

A geometric proof diagram showing a circle with chord AB, its perpendicular bisector, and two right triangles formed
Geometric proof using congruent right triangles

Let's look at a simple way to understand why this theorem is true. We'll use some basic geometry:

Imagine a circle with center O and a chord AB.
Let M be the midpoint of chord AB (so AM = MB).
Draw line OM. This is the line from the center to the midpoint of the chord.
Now consider triangles OAM and OBM.
OA = OB (both are radii of the circle, so they're equal).
AM = MB (because M is the midpoint).
OM = OM (it's the same line in both triangles).
So by the Side-Side-Side rule, triangles OAM and OBM are congruent.
This means that angle OMA = angle OMB.
But these two angles together form a straight line (180 degrees).
So each must be 90 degrees, making OM perpendicular to AB.
Therefore, OM is the perpendicular bisector of chord AB.

This shows that the line from the center to the midpoint of any chord is always perpendicular to that chord. That's the proof!

Examples of the Theorem

Three different circles with chords of different lengths, each with their perpendicular bisectors passing through the center
No matter the chord's length or position, its perpendicular bisector always goes through the center

Let's look at some examples to understand how this theorem works in different situations:

Equal Chords

Chords that are equal in length are equidistant from the center of the circle.

Longest Chord

The diameter is the longest chord, and its perpendicular bisector is any line through the center.

Finding the Center

To find a circle's center, draw two chords and their perpendicular bisectors. Where they cross is the center!

This theorem is very useful for solving geometry problems involving circles. Architects and engineers use this principle when designing circular structures!

Geometry Quiz

Test your knowledge with this quiz! Answer all 5 questions to see how much you've learned.

1. What is a chord of a circle?
2. What does a perpendicular bisector do?
3. According to the theorem, where does the perpendicular bisector of a chord always pass through?
4. What is the longest possible chord in a circle called?
5. How can you find the center of a circle using this theorem?

Frequently Asked Questions

Here are answers to some common questions about the perpendicular bisector of a chord:

Math Trivia

Discover some interesting facts about circles and geometry!

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