
Table of Contents
 Proving that Opposite Sides of a Quadrilateral Circumscribe a Circle
 Understanding Circumscribed Circles
 Conditions for a Quadrilateral to Circumscribe a Circle
 Proof: Opposite Sides of a Cyclic Quadrilateral Circumscribe a Circle
 Step 1: Consider a Cyclic Quadrilateral
 Step 2: Connect the Midpoints of Opposite Sides
 Step 3: Prove that the Line Segments are Concurrent
 Step 4: Prove that O is the Center of the Circumscribed Circle
 Key Takeaways
 Q&A
 1. Can all quadrilaterals circumscribe a circle?
 2. Are all cyclic quadrilaterals convex?
 3. What are some reallife applications of circumscribed circles?
Quadrilaterals are fascinating geometric shapes that have been studied for centuries. One interesting property of certain quadrilaterals is that their opposite sides can circumscribe a circle. In this article, we will explore the concept of a circumscribed circle, understand the conditions under which a quadrilateral can circumscribe a circle, and provide a proof for this intriguing property.
Understanding Circumscribed Circles
Before delving into the specifics of quadrilaterals, let’s first understand the concept of a circumscribed circle. A circumscribed circle is a circle that passes through all the vertices of a given polygon. In other words, the polygon is inscribed within the circle, and the circle touches all its vertices.
For example, consider a triangle. If a circle can be drawn such that it passes through all three vertices of the triangle, then that circle is the circumscribed circle for that triangle. The center of the circle is called the circumcenter, and it is equidistant from all the vertices of the triangle.
Conditions for a Quadrilateral to Circumscribe a Circle
Now that we understand the concept of a circumscribed circle, let’s explore the conditions under which a quadrilateral can circumscribe a circle. A quadrilateral can circumscribe a circle if and only if it is a cyclic quadrilateral.
A cyclic quadrilateral is a quadrilateral whose vertices lie on a single circle. In other words, all four vertices of the quadrilateral can be connected to form a circle that passes through each vertex. This property is also known as the “inscribed quadrilateral” property.
There are several equivalent conditions for a quadrilateral to be cyclic:
 The opposite angles of the quadrilateral are supplementary.
 The sum of any pair of opposite angles is 180 degrees.
 The diagonals of the quadrilateral intersect at a right angle.
 The perpendicular bisectors of the sides of the quadrilateral are concurrent.
These conditions ensure that the quadrilateral can be inscribed within a circle, and hence, its opposite sides can circumscribe a circle.
Proof: Opposite Sides of a Cyclic Quadrilateral Circumscribe a Circle
Now, let’s prove the statement that the opposite sides of a cyclic quadrilateral circumscribe a circle. We will use the properties of cyclic quadrilaterals to establish this result.
Step 1: Consider a Cyclic Quadrilateral
Let ABCD be a cyclic quadrilateral, where the vertices A, B, C, and D lie on a single circle.
Step 2: Connect the Midpoints of Opposite Sides
Draw the line segments connecting the midpoints of the opposite sides of the quadrilateral. Let E be the midpoint of AB, F be the midpoint of BC, G be the midpoint of CD, and H be the midpoint of DA.
Step 3: Prove that the Line Segments are Concurrent
Claim: The line segments EG and FH intersect at a single point O.
Proof: Since ABCD is a cyclic quadrilateral, the diagonals AC and BD intersect at a right angle. Let the point of intersection be P.
Now, consider triangle ACP. The line segment EG is the midline of triangle ACP, and the line segment FH is also the midline of triangle ACP. Therefore, EG and FH are parallel to each other.
Since EG and FH are parallel, and they intersect at point P, we can conclude that EG and FH are concurrent at a single point O.
Step 4: Prove that O is the Center of the Circumscribed Circle
Claim: The point O is the center of the circle that circumscribes the quadrilateral ABCD.
Proof: Since EG and FH are concurrent at point O, O is equidistant from the midpoints of the opposite sides of the quadrilateral.
Let M be the midpoint of AD. Since O is equidistant from E and H, we can conclude that OM is perpendicular to AD.
Similarly, we can prove that OM is perpendicular to BC, AB, and CD. Therefore, O is equidistant from all the vertices of the quadrilateral.
Thus, O is the center of the circle that circumscribes the quadrilateral ABCD.
Key Takeaways
By understanding the concept of a circumscribed circle and the conditions for a quadrilateral to be cyclic, we have proven that the opposite sides of a cyclic quadrilateral circumscribe a circle. This property is a fascinating result in geometry and has numerous applications in various fields.
Some key takeaways from this article include:
 A circumscribed circle is a circle that passes through all the vertices of a given polygon.
 A quadrilateral can circumscribe a circle if and only if it is a cyclic quadrilateral.
 Conditions for a quadrilateral to be cyclic include supplementary opposite angles, the sum of any pair of opposite angles being 180 degrees, intersecting diagonals at a right angle, and concurrent perpendicular bisectors of the sides.
 The opposite sides of a cyclic quadrilateral intersect at a single point, which is the center of the circumscribed circle.
Understanding the properties and characteristics of quadrilaterals and their circumscribed circles not only enhances our knowledge of geometry but also provides a foundation for solving complex geometric problems in various fields of study.
Q&A
1. Can all quadrilaterals circumscribe a circle?
No, not all quadrilaterals can circumscribe a circle. Only cyclic quadrilaterals, where the vertices lie on a single circle, can circumscribe a circle.
2. Are all cyclic quadrilaterals convex?
No, not all cyclic quadrilaterals are convex. There are also nonconvex cyclic quadrilaterals where one or more of the interior angles are greater than 180 degrees.
3. What are some reallife applications of circumscribed circles?
Circumscribed circles have various applications in reallife scenarios, including:
 In architecture and construction, circumscribed circles help determine the optimal placement of columns and pillars in buildings.
 In navigation, circumscribed circles are used to calculate the position and distance between multiple points on a map.
 In computer graphics