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Arcs in Circles

Sections of a circle and central angles.

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Arcs in Circles

What if the Ferris wheel below had equally spaced seats, such that the central angle were \begin{align*}20^\circ\end{align*}. How many seats are there? Why do you think it is important to have equally spaced seats on a Ferris wheel?

If the radius of this Ferris wheel is 25 ft., how far apart are two adjacent seats? Round your answer to the nearest tenth. The shortest distance between two points is a straight line. .

Arcs in Circles 

A central angle is the angle formed by two radii of the circle with its vertex at the center of the circle. In the picture below, the central angle would be \begin{align*}\angle BAC\end{align*}. Every central angle divides a circle into two arcs (an arc is a section of the circle). In this case the arcs are \begin{align*}\widehat{BC}\end{align*} and \begin{align*}\widehat{BDC}\end{align*}. Notice the arc above the letters. To label an arc, always use this curve above the letters. Do not confuse \begin{align*}\overline{BC}\end{align*} and \begin{align*}\widehat{BC}\end{align*}.

If \begin{align*}D\end{align*} was not on the circle, we would not be able to tell the difference between \begin{align*}\widehat{BC}\end{align*} and \begin{align*}\widehat{BDC}\end{align*}. There are \begin{align*}360^\circ\end{align*} in a circle, where a semicircle is half of a circle, or \begin{align*}180^\circ\end{align*}. \begin{align*}m \angle EFG = 180^\circ\end{align*}, because it is a straight angle, so \begin{align*}m \widehat{EHG}= 180^\circ\end{align*} and \begin{align*}m \widehat{EJG} = 180^\circ\end{align*}.

  • Semicircle: An arc that measures \begin{align*}180^\circ\end{align*}.
  • Minor Arc: An arc that is less than \begin{align*}180^\circ\end{align*}.
  • Major Arc: An arc that is greater than \begin{align*}180^\circ\end{align*}. Always use 3 letters to label a major arc.

Two arcs are congruent if their central angles are congruent. The measure of the arc formed by two adjacent arcs is the sum of the measures of the two arcs (Arc Addition Postulate). An arc can be measured in degrees or in a linear measure (cm, ft, etc.). In this chapter we will use degree measure. The measure of the minor arc is the same as the measure of the central angle that corresponds to it. The measure of the major arc equals to \begin{align*}360^\circ\end{align*} minus the measure of the minor arc. In order to prevent confusion, major arcs are always named with three letters; the letters that denote the endpoints of the arc and any other point on the major arc. When referring to the measure of an arc, always place an “\begin{align*}m\end{align*}” in from of the label.

Measuring Arcs

Find \begin{align*}m\widehat{AB}\end{align*} and \begin{align*}m\widehat{ADB}\end{align*} in \begin{align*}\bigodot C\end{align*}.

\begin{align*}m\widehat{AB}= m\angle{ACB}\end{align*}. So, \begin{align*}m\widehat{AB}= 102^\circ\end{align*}.

\begin{align*}m\widehat{ADB}=360^\circ - m\widehat{AB}=360^\circ-102^\circ=258^\circ\end{align*}

Identifying and Measuring Minor Arcs 

Find the measures of the minor arcs in \begin{align*}\bigodot{A}\end{align*}. \begin{align*}\overline{EB}\end{align*} is a diameter.

Because \begin{align*}\overline{EB}\end{align*} is a diameter, \begin{align*}m\angle EAB=180^\circ\end{align*}. Each arc has the same measure as its corresponding central angle.

\begin{align*}m \widehat{BF} & = m \angle FAB = 60^\circ\\ m\widehat{EF} & = m \angle EAF = 120^\circ \rightarrow 180^\circ - 60^\circ\\ m\widehat{ED} & = m \angle EAD = 38^\circ \ \rightarrow 180^\circ - 90^\circ - 52^\circ\\ m\widehat{DC} & = m \angle DAC = 90^\circ\\ m\widehat{BC} & = m \angle BAC = 52^\circ\end{align*}

Using the Arc Addition Postulate 

Find the measures of the indicated arcs in \begin{align*}\bigodot A\end{align*}. \begin{align*}\overline{EB}\end{align*} is a diameter.

Use the Arc Addition Postulate. 

a) \begin{align*}m\widehat{FED}\end{align*}

\begin{align*}m\widehat{FED} = m\widehat{FE} +m\widehat{ED} = 120^\circ+38^\circ=158^\circ\end{align*}

b) \begin{align*}m\widehat{CDF}\end{align*}

\begin{align*}m\widehat{CDF} = m\widehat{CD} + m \widehat{DE} + m \widehat{EF} = 90^\circ + 38^\circ + 120^\circ = 248^\circ\end{align*}

c) \begin{align*}m\widehat{DFC}\end{align*}

\begin{align*}m \widehat{DFC} = m\widehat{ED} + m\widehat{EF} + m\widehat{FB} + m\widehat{BC} = 38^\circ + 120^\circ + 60^\circ + 52^\circ = 270^\circ\end{align*}

Ferris Wheel Problem Revisited

Because the seats are \begin{align*}20^\circ\end{align*} apart, there will be \begin{align*}\frac{360^\circ}{20^\circ}=18\end{align*} seats. It is important to have the seats evenly spaced for balance. To determine how far apart the adjacent seats are, use the triangle to the right. We will need to use sine to find \begin{align*}x\end{align*} and then multiply it by 2.

\begin{align*}\sin 10^\circ &= \frac{x}{25}\\ x = 25 \sin 10^\circ &= 4.3 \ ft.\end{align*}

The total distance apart is 8.6 feet.


Example 1

List the congruent arcs in \begin{align*}\bigodot C\end{align*} below. \begin{align*}\overline{AB}\end{align*} and \begin{align*}\overline{DE}\end{align*} are diameters.

\begin{align*}\angle ACD \cong \angle ECB\end{align*} because they are vertical angles. \begin{align*}\angle DCB \cong \angle ACE\end{align*} because they are also vertical angles.

\begin{align*}\widehat{AD} \cong \widehat{EB}\end{align*} and \begin{align*}\widehat{AE} \cong \widehat{DB}\end{align*} 

Example 2

Are the blue arcs congruent? Explain why or why not.


\begin{align*}\widehat{AD} \cong \widehat{BC}\end{align*} because they have the same central angle measure and are in the same circle.


The two arcs have the same measure, but are not congruent because the circles have different radii.

Example 3

Find the value of \begin{align*}x\end{align*} for \begin{align*}\bigodot C\end{align*} below.

The sum of the measure of the arcs is \begin{align*}360^\circ\end{align*} because they make a full circle.

\begin{align*}m \widehat{AB} + m \widehat{AD} + m \widehat{DB} & = 360^\circ\\ (4x+15)^\circ+92^\circ+(6x+3)^\circ&=360^\circ\\ 10x+110^\circ&=360^\circ\\ 10x&=250\\ x&=25\end{align*}


Determine if the arcs below are a minor arc, major arc, or semicircle of \begin{align*}\bigodot G\end{align*}. \begin{align*}\overline{EB}\end{align*} is a diameter.

  1. \begin{align*}\widehat{AB}\end{align*}
  2. \begin{align*}\widehat{ABD}\end{align*}
  3. \begin{align*}\widehat{BCE}\end{align*}
  4. \begin{align*}\widehat{CAE}\end{align*}
  5. \begin{align*}\widehat{ABC}\end{align*}
  6. \begin{align*}\widehat{EAB}\end{align*}
  7. Are there any congruent arcs? If so, list them.
  8. If \begin{align*}m \widehat{BC}=48^\circ\end{align*}, find \begin{align*}m \widehat{CD}\end{align*}.
  9. Using #8, find \begin{align*}m \widehat{CAE}\end{align*}.

Determine if the blue arcs are congruent. If so, state why.

Find the measure of the indicated arcs or central angles in \begin{align*}\bigodot A\end{align*}. \begin{align*}\overline{DG}\end{align*} is a diameter.

  1. \begin{align*}\widehat{DE}\end{align*}
  2. \begin{align*}\widehat{DC}\end{align*}
  3. \begin{align*}\angle GAB\end{align*}
  4. \begin{align*}\widehat{FG}\end{align*}
  5. \begin{align*}\widehat{EDB}\end{align*}
  6. \begin{align*}\angle EAB\end{align*}
  7. \begin{align*}\widehat{DCF}\end{align*}
  8. \begin{align*}\widehat{DBE}\end{align*}

Algebra Connection Find the measure of \begin{align*}x\end{align*} in \begin{align*}\bigodot P\end{align*}.

  1. What can you conclude about \begin{align*}\bigodot A\end{align*} and \begin{align*}\bigodot B\end{align*}?

Review (Answers)

To view the Review answers, open this PDF file and look for section 9.3. 

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A single section of the circle, that describes a particular angle.

central angle

An angle formed by two radii and whose vertex is at the center of the circle.

major arc

An arc that is greater than 180^\circ.

minor arc

An arc that is less than 180^\circ.


An arc that measures 180^\circ.

Arc Addition Postulate

Arc addition postulate states that the measure of the arc formed by two adjacent arcs is the sum of the measures of the two arcs.


Diameter is the measure of the distance across the center of a circle. The diameter is equal to twice the measure of the radius.

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