Properties of Chords

Properties of a chord in a circle

Before discussing the properties of a chord in a circle, let's quickly consider the definition of chord. A chord is a line segment that passes from any two points in a circle. A chord can be drawn with its two endpoints anywhere on the circle.

When a chord passes through the center of the circle, we refer to it as the circle's diameter. A diameter divides a circle into two semicircles, whereas any other chord splits a circle into a major arc and a minor arc.

Fig. 1. Chords of the circle.

The basic properties of a chord are as follows:

• The chord divides a circle into two segments: major segment and minor segment.

• A perpendicular line bisects the chord if drawn from the center of the circle to the chord.

• Two equal chords have two equal angles subtended at the center of the circle.

• Chords with equal length are equidistant from the center of the circle.

Now let's dive deeper and understand the properties of chords more clearly.

Properties of chords: Perpendicular bisectors

Suppose that you have a chord $AB$ on a circle having a center $O$, which the figure below illustrates. If we draw a line from the center $O$ to the point $P$ on the chord $AB$, $OP$ is perpendicular to $AB$, and $OP$ bisects $AB$. In other words, $OP$ is the perpendicular bisector of $AB$ such that $AP$ and $PB$ are congruent. Hence,

$$\text{if}\overline{OP}\perp \overline{AB}\text{, then}\overline{AP}\cong \overline{PB}$$

Fig. 2. Chord with perpendicular bisector.

Determining the diameter by a perpendicular bisector

Have a look at the below figure. $CD$ is the chord that acts as a perpendicular bisector to the chord $AB$. For this scenario,

$$\text{if}\overline{CD}\perp \overline{AB}\text{, then}\overline{CD}\text{is a diameter of the circle}$$

Fig. 3. Chord with perpendicular bisector.

Properties of congruent chords

If two chords are equidistant from the circle's center, we know that they must be congruent. This property of chords is depicted in the figure below: chords $AB$ and $DE$ are equidistant on the circle. Also note that $CF$ and $CG$ are equal in length (congruent). The equal length of these line segments $CF$ and $CG$ help us confirm that the two chords $AB$ and $DE$ are of equal distance from the circle's center.

$$\text{if}\overline{CF}\cong \overline{CG}\text{, then}\overline{AB}\cong \overline{DE}$$

Fig. 4. Chords with equal lines from center.

Now, let's say we are given the information that $CF$ is perpendicular to $AB$, and $CG$ is perpendicular to $DE$. In this case, we can use the property of perpendicular bisectors to make the following conclusions about the chords: $$\text{if}\overline{CF}\perp \overline{AB}\text{and}\overline{CG}\perp \overline{DE}\text{and}\overline{CF}=\overline{CG}\text{, then}\overline{AF}=\overline{FB}=\overline{DG}=\overline{GE}$$

Fig. 5. Perpendicular chords with equal sides.

Properties of intersecting chords

The intersecting chords theorem states that when chords in a circle intersect, the products of their segments' lengths are equal. Have a look at the figure below, where two chords $RS$ and $PQ$ intersect at point $A$, with $O$ as the center of the circle. So, we can write the chord theorem as:

$$(\overline{SA})\cdot (\overline{AR})=(\overline{PA})\cdot (\overline{AQ})$$

Fig. 6. Intersecting chords.

Properties of chords: Subtended angles

The next property of chords we'll discuss deals with subtended angles. First, let's clarify the meaning of a subtended angle. When the two endpoints of a chord are joined (using line segments) to form an angle located at a point outside of that chord, that angle is considered the subtended angle.

For example, suppose $AB$ is a chord and $C$ is a point outside the chord in a circle. Then $\mathrm{\angle }ACB$ is the subtended angle.

Fig. 7. Chord with subtended angle.

Now, let's consider the next chords property by having a look at the figure below: in this figure, two equal chords subtend angles at the center of a circle. In this case, according to the properties of chords, both subtended angles are equal.

$$\Rightarrow \mathrm{\angle }AOB=\mathrm{\angle }DOC$$

Fig. 8. Two equal chords subtend equal angles at the center.

Properties of chords: Calculation of length with formulas

In certain circumstances, we are able to calculate the length of a chord using formulas, including:

• When the chord's subtended angle at the center of the circle is provided.
• When the radius and distance from the chord to the center is given.

These circumstances are shown in the figure below. Suppose for the chord $CB$ on the circle with center $A$, $r$ is the radius, $d$ is the distance from the chord to the center, and $\theta$ is the subtended angle.

Fig. 9. Chord length with radius or subtended angle.

The length of chord $CB$ shown in the figure can be calculated by use of the following formulas:

When the subtended angle is given, then:

$$\text{Chord}=2\times r\times \sin \left(\frac{\theta }{2}\right)$$

If the radius and distance from the center to the chord is given, then:

$$\text{Chord}=2\times \sqrt{r^2-d^2}$$

Properties of chords examples

Let's exercise our knowledge of the properties of chords with some example problems.