Inscribed Quadrilaterals in Circles
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
- 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
∠Band ∠Dso 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.
Solving for Unknown Values
1. Find the value of the missing variable.
2. Find the value of the missing variable.
It is easiest to figure out
The opposite angles are supplementary. Set up an equation for
First, note that
Fill in the blanks.
(n)_______________ polygon has all its vertices on a circle.
- The _____________ angles of an inscribed quadrilateral are ________________.
m∠DBC mBCˆ mABˆ m∠ACD m∠ADC m∠ACB
Find the value of
Use the diagram below to find the measures of the indicated angles and arcs in problems 14-19.
m∠EBO m∠EOB mBCˆ m∠ABO m∠A m∠EDC
To view the Review answers, open this PDF file and look for section 9.6.