In Euclidean geometry, a cyclic parallelogram is a special type of quadrilateral where all four vertices lie on a common circle. One of the most interesting properties of a cyclic parallelogram is that it can be proven to be a rectangle. This proof involves utilizing various geometric properties and theorems to demonstrate that the angles of the cyclic parallelogram are right angles, thereby confirming it as a rectangle.
Understanding the Properties of Cyclic Parallelograms
Before diving into the proof, let’s establish some foundational knowledge about cyclic parallelograms. A parallelogram is a quadrilateral with opposite sides that are parallel. In a cyclic parallelogram, the additional property of all vertices lying on a common circle creates a unique geometric configuration. This relationship with a circle allows us to leverage the angles and properties associated with cyclic quadrilaterals.
Proof Outline
To prove that a cyclic parallelogram is a rectangle, we will follow these steps:

Draw the Diagram: Start by sketching the cyclic parallelogram, ensuring that all vertices lie on the circle. Label the vertices and any given angle measures.

Establish Parallelogram Properties: Recall that in a parallelogram, opposite sides are parallel. Use this property to identify pairs of parallel sides within the cyclic parallelogram.

Leverage Opposite Angles of a Parallelogram: In a parallelogram, opposite angles are equal. Utilize this property to find relationships between the angles in the cyclic parallelogram.

Utilize Cyclic Quadrilateral Properties: Since the vertices lie on a circle, the opposite angles of the cyclic parallelogram are supplementary. Use this information to derive more angle relationships.

Show Angle Measures: By combining the properties of parallelograms and cyclic quadrilaterals, demonstrate that the angles in the cyclic parallelogram are all right angles, which is a defining characteristic of a rectangle.
Detailed Proof
Let’s delve deeper into the detailed proof of why a cyclic parallelogram is indeed a rectangle.
1. Start by Understanding Parallelogram Properties
Consider a cyclic parallelogram ABCD, where the vertices A, B, C, and D lie on a common circle. Given that it is a parallelogram, we know that:
 AB is parallel to DC (opposite sides are parallel)
 BC is parallel to AD
2. Explore Opposite Angles in a Parallelogram
In a parallelogram, opposite angles are equal. Let’s denote the angles in cyclic parallelogram ABCD as follows:
– ∠A = α
– ∠B = β
– ∠C = γ
– ∠D = δ
Given the parallelogram properties, we know that:
– ∠A = ∠C = α (opposite angles are equal)
– ∠B = ∠D = β (opposite angles are equal)
3. Utilize Properties of Cyclic Quadrilaterals
Since ABCD is a cyclic quadrilateral, we can apply the properties associated with such figures. In a cyclic quadrilateral, opposite angles are supplementary. Therefore:
– ∠A + ∠C = 180°
– ∠B + ∠D = 180°
Substitute the values of ∠A and ∠C with α:
– α + α = 180°
– 2α = 180°
– α = 90°
Similarly, substituting the values of ∠B and ∠D with β:
– β + β = 180°
– 2β = 180°
– β = 90°
4. Prove that ABCD is a Rectangle
As we have shown that α = 90° and β = 90°, it is evident that all angles in the cyclic parallelogram ABCD are right angles. This implies that ABCD is a rectangle, as a rectangle is defined by having all internal angles measuring 90°.
FAQs
 What is a cyclic parallelogram?

A cyclic parallelogram is a quadrilateral where all four vertices lie on a common circle.

How is a parallelogram defined?

A parallelogram is a quadrilateral with opposite sides that are parallel.

Why are opposite angles in a parallelogram equal?

Opposite angles in a parallelogram are equal due to the properties of parallel lines and angles.

What is the significance of a cyclic quadrilateral in geometry?

Cyclic quadrilaterals have special properties related to angles formed by their vertices lying on a circle.

How do you prove a cyclic parallelogram is a rectangle?
 By demonstrating that all angles in the cyclic parallelogram are right angles through the properties of parallelograms and cyclic quadrilaterals.
In conclusion, the proof presented showcases the geometric elegance of cyclic parallelograms and their relationship with rectangles. By understanding the properties of parallelograms and utilizing the properties of cyclic quadrilaterals, we can definitively prove that a cyclic parallelogram is indeed a rectangle.