What if you were given a circle with a quadrilateral inscribed in it? How could you use information about the arcs formed by the quadrilateral and/or the quadrilateral's angle measures to find the measure of the unknown quadrilateral angles? After completing this Concept, you'll be able to apply the Inscribed Quadrilateral Theorem to solve problems like this one.
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CK12 Foundation: Chapter9InscribedQuadrilateralsinCirclesA
Guidance
An inscribed polygon is a polygon where every vertex is on a circle. Note, that not every quadrilateral or polygon can be inscribed in a circle. Inscribed quadrilaterals are also called cyclic quadrilaterals. For these types of quadrilaterals, they must have one special property. We will investigate it here.
Investigation: Inscribing Quadrilaterals
Tools Needed: pencil, paper, compass, ruler, colored pencils, scissors
 Draw a circle. Mark the center point
A .  Place four points on the circle. Connect them to form a quadrilateral. Color the 4 angles of the quadrilateral 4 different colors.
 Cut out the quadrilateral. Then cut the quadrilateral into two triangles, by cutting on a diagonal.
 Line up
∠B and∠D so that they are adjacent angles. What do you notice? What does this show?
This investigation shows that the opposite angles in an inscribed quadrilateral are supplementary. By cutting the quadrilateral in half, through the diagonal, we were able to show that the other two angles (that we did not cut through) formed a linear pair when matched up.
Inscribed Quadrilateral Theorem: A quadrilateral is inscribed in a circle if and only if the opposite angles are supplementary.
Example A
Find the value of the missing variable.
Example B
Find the value of the missing variable.
It is easiest to figure out
Example C
Find
The opposite angles are supplementary. Set up an equation for
Watch this video for help with the Examples above.
CK12 Foundation: Chapter9InscribedQuadrilateralsinCirclesB
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 an angle formed by two radii and whose 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. An inscribed polygon is a polygon where every vertex is on the circle.
Guided Practice
Quadrilateral

m∠A 
m∠B 
m∠C 
m∠D
Answers:
First, note that
1.
2.
3.
4.
Interactive Practice
Practice
Fill in the blanks.
 A
(n) _______________ polygon has all its vertices on a circle.  The _____________ angles of an inscribed quadrilateral are ________________.
Quadrilateral

m∠DBC 
mBCˆ 
mABˆ 
m∠ACD 
m∠ADC 
m∠ACB
Find the value of
Solve for
Use the diagram below to find the measures of the indicated angles and arcs in problems 1419.

m∠EBO 
m∠EOB 
mBCˆ 
m∠ABO 
m∠A 
m∠EDC