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Angles Outside a Circle

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What if you wanted to figure out the angle at which the sun's rays hit the earth? The sun’s rays hit the earth such that the tangent rays determine when daytime and night time are. The time and Earth’s rotation determine when certain locations have sun. If the arc that is exposed to sunlight is 178^\circ , what is the angle at which the sun’s rays hit the earth (x^\circ) ? After completing this Concept, you'll be able to use properties of angles created by tangent lines to answer this question.

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CK-12 Foundation: Chapter9AnglesOutsideaCircleA

Learn more about secants by watching the second half of this video.


An angle is considered to be outside a circle if the vertex of the angle is outside the circle and the sides are tangents or secants. There are three types of angles that are outside a circle: an angle formed by two tangents, an angle formed by a tangent and a secant, and an angle formed by two secants. Just like an angle inside or on a circle, an angle outside a circle has a specific formula, involving the intercepted arcs.

Investigation: Find the Measure of an Angle outside a Circle

Tools Needed: pencil, paper, ruler, compass, protractor, colored pencils (optional)

  1. Draw three circles and label the centers A, B , and C . In \bigodot A draw two secant rays with the same endpoint, \overrightarrow{DE} and \overrightarrow{DF} . In \bigodot B , draw two tangent rays with the same endpoint, \overrightarrow{LM} and \overrightarrow{LN} . In \bigodot C , draw a tangent ray and a secant ray with the same endpoint, \overrightarrow{QR} and \overrightarrow{QS} . Label the points of intersection with the circles like they are in the pictures below.
  2. Draw in all the central angles: \angle GAH, \angle EAF, \angle MBN, \angle RCT, \angle RCS . Then, find the measures of each of these angles using your protractor. Use color to differentiate.
  3. Find m \angle EDF, m \angle MLN , and m \angle RQS .
  4. Find \frac{m \widehat{EF}-m \widehat{GH}}{2}, \frac{m \widehat{MPN}-m \widehat{MN}}{2} , and \frac{m \widehat{RS}-m \widehat{RT}}{2} . What do you notice?

Outside Angle Theorem: The measure of an angle formed by two secants, two tangents, or a secant and a tangent drawn from a point outside the circle is equal to half the difference of the measures of the intercepted arcs.

Example A

Find the value of x . You may assume lines that look tangent, are.

Set up an equation using the Outside Angle Theorem.

\frac{(5x+10)^\circ-(3x+4)^\circ}{2} &= 30^\circ\\(5x+10)^\circ-(3x+4)^\circ &= 60^\circ\\5x+10^\circ-3x-4^\circ &= 60^\circ\\2x+6^\circ &= 60^\circ\\2x &= 54^\circ\\x &= 27^\circ

Example B

Find the value of x .

x=\frac{120^\circ - 32^\circ}{2}=\frac{88^\circ}{2}=44^\circ .

Example C

Find the value of x .

First note that the missing arc by angle x measures 32^\circ because the complete circle must make 360^\circ . Then, x=\frac{141^\circ - 32^\circ}{2}=\frac{109^\circ}{2}=54.5^\circ .

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter9AnglesOutsideaCircleB

Concept Problem Revisited

If 178^\circ of the Earth is exposed to the sun, then the angle at which the sun’s rays hit the Earth is 2^\circ . From the Outside Angle Theorem, these two angles are supplementary. From this, we also know that the other 182^\circ of the Earth is not exposed to sunlight and it is probably night time.


A circle is the set of all points that are the same distance away from a specific point, called the center . A radius is the distance from the center to the circle. A chord is a line segment whose endpoints are on a circle. A diameter is a chord that passes through the center of the circle. The length of a diameter is two times the length of a radius. A central angle is the angle formed by two radii and whose its vertex is at the center of the circle. An inscribed angle is an angle with its vertex on the circle and whose sides are chords. The intercepted arc is the arc that is inside the inscribed angle and whose endpoints are on the angle. A tangent is a line that intersects a circle in exactly one point. The point of tangency is the point where the tangent line touches the circle. A secant is a line that intersects a circle in two points.

Guided Practice

Find the measure of x .





For all of the above problems we can use the Outside Angle Theorem.

1. x=\frac{125^\circ-27^\circ}{2}=\frac{98^\circ}{2}=49^\circ

2. 40^\circ is not the intercepted arc. Be careful! The intercepted arc is 120^\circ , (360^\circ-200^\circ-40^\circ) . Therefore, x=\frac{200^\circ-120^\circ}{2}=\frac{80^\circ}{2}=40^\circ .

3. First, we need to find the other intercepted arc, 360^\circ-265^\circ=95^\circ . x=\frac{265^\circ-95^\circ}{2}=\frac{170^\circ}{2}=85^\circ

Interactive Practice

Explore More

Find the value of the missing variable(s).

Solve for x .

  1. Prove the Outside Angle Theorem

Given : Secant rays \overrightarrow{AB} and \overrightarrow{AC}

Prove : m\angle a = \frac{1}{2} \left (m\widehat{BC}-m\widehat{DE} \right )

  1. Draw two secants that intersect:
    1. inside a circle.
    2. on a circle.
    3. outside a circle.

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