9.8: Angles Outside a Circle
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 , what is the angle at which the sun’s rays hit the earth ? After completing this Concept, you'll be able to use properties of angles created by tangent lines to answer this question.
Watch This
CK12 Foundation: Chapter9AnglesOutsideaCircleA
Watch the last part of this video.
Guidance
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)
 Draw three circles and label the centers , and . In draw two secant rays with the same endpoint, and . In , draw two tangent rays with the same endpoint, and . In , draw a tangent ray and a secant ray with the same endpoint, and . Label the points of intersection with the circles like they are in the pictures below.
 Draw in all the central angles: . Then, find the measures of each of these angles using your protractor. Use color to differentiate.
 Find , and .
 Find , and . 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 . You may assume lines that look tangent, are.
Set up an equation using the Outside Angle Theorem.
Example B
Find the value of .
.
Example C
Find the value of .
First note that the missing arc by angle measures because the complete circle must make . Then, .
Watch this video for help with the Examples above.
CK12 Foundation: Chapter9AnglesOutsideaCircleB
Concept Problem Revisited
If of the Earth is exposed to the sun, then the angle at which the sun’s rays hit the Earth is . From the Outside Angle Theorem, these two angles are supplementary. From this, we also know that the other of the Earth is not exposed to sunlight and it is probably night time.
Vocabulary
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 .
1.
2.
3.
Answers:
For all of the above problems we can use the Outside Angle Theorem.
1.
2. is not the intercepted arc. Be careful! The intercepted arc is , . Therefore, .
3. First, we need to find the other intercepted arc, .
Practice
Find the value of the missing variable(s).
Solve for .
 Prove the Outside Angle Theorem
Given : Secant rays and
Prove :

Draw two secants that intersect:
 inside a circle.
 on a circle.
 outside a circle.
central angle
An angle formed by two radii and whose vertex is at the center of the circle.chord
A line segment whose endpoints are on a circle.diameter
A chord that passes through the center of the circle. The length of a diameter is two times the length of a radius.inscribed angle
An angle with its vertex on the circle and whose sides are chords.intercepted arc
The arc that is inside an inscribed angle and whose endpoints are on the angle.point of tangency
The point where the tangent line touches the circle.Image Attributions
Description
Learning Objectives
Here you'll learn how to calculate angles formed outside a circle by tangent and secant lines.