10.2: Inscribed Angles Theorem
This activity is intended to supplement Geometry, Chapter 9, Lesson 4.
ID: 12437
Time Required: 15 minutes
Activity Overview
Students will begin this activity by looking at inscribed angles and central angles and work towards discovering a relationship among the two, the Inscribed Angle Theorem. Then, students will look at two corollaries to the theorem.
Topic: Circles
 Construct central and inscribed angles
 Inscribed angles theorem
Teacher Preparation and Notes
 This activity was written to be explored with the Cabri Jr. application on the TI84.
 To download Cabri Jr, go to http://www.education.ti.com/calculators/downloads/US/Software/Detail?id=258#.
 To download the calculator files, go to http://www.education.ti.com/calculators/downloads/US/Activities/Detail?id=12437 and select INSCRIB1INSCRIB5.
Associated Materials
 Student Worksheet: Inscribed Angle Theorem http://www.ck12.org/flexr/chapter/9694, scroll down to the second activity.
 Cabri Jr. Application
 INSCRIB1.8xv, INSCRIB2.8xv, INSCRIB3.8xv, INSCRIB4.8xv, and INSCRIB5.8xv
Problem 1 – Similar Triangles
Students will begin this activity by looking at inscribed angles and central angles and work towards discovering a relationship among the two.
Students will be asked to collect data by moving points
On page 1.6, students will look at two inscribed angles intercepted by the same arc and are asked to make a conjecture about the relationship.
In an advanced setting a proof of the inscribed angle theorem and the two conjectures in problem one are appropriate and can be proved using isosceles triangles.
Problem 2 – Extension of the Inscribed Angle Theorem
In Problem 2, students will look at two more angles created from the central angle and the intercepted arc. Both sections of this problem are corollaries of the Inscribed Angle Theorem and both solutions are congruent to the measure of the central angle intercepted by the arc or onehalf the measure of the central angle.
Solutions
1. Sample answers
Position 
Measure of 
Measure of 


1 



2 



3 



4 



2.
3. Sample answers
Position 
Measure of 
Measure of 

1 


2 


3 


4 


4. Sample answer: They are congruent.
5. diameter of the circle
6.
7. Sample answers
Position 
Measure of 
Measure of 
Measure of 

1 



2 



3 



4 



8.
9. Sample answers
Position 
Measure of 
Measure of 
Measure of 

1  \begin{align*}57.65^\circ\end{align*}  \begin{align*}115.30^\circ\end{align*}  \begin{align*}57.64^\circ\end{align*} 
2  \begin{align*}49.20^\circ\end{align*}  \begin{align*}98.40^\circ\end{align*}  \begin{align*}49.19^\circ\end{align*} 
3  \begin{align*}54.90^\circ\end{align*}  \begin{align*}109.80^\circ\end{align*}  \begin{align*}54.90^\circ\end{align*} 
4  \begin{align*}63.55^\circ\end{align*}  \begin{align*}127.10^\circ\end{align*}  \begin{align*}63.54^\circ\end{align*} 
10. \begin{align*}\frac{1}{2}\end{align*}
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