8.7 Practice Question
Duration: 2 min
This video lesson is available to enrolled students.
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This educational video segment focuses on solving a graph theory practice problem involving simple non-directed graphs. The instructor presents a question asking for the possible number of vertices in a graph with 24 edges where every vertex has an identical degree K. The core concept applied is the Handshaking Lemma, which states that the sum of all vertex degrees equals twice the number of edges. The instructor demonstrates how to translate this theorem into an algebraic equation by defining |V| = x for the number of vertices and deg(v) = K for the degree. Through this derivation, he establishes that x * K must equal 48 (since 24 edges multiplied by 2 equals 48). The solution process involves identifying that the number of vertices x must be a divisor of 48. By testing the provided multiple-choice options (20, 15, 10, and 8), the instructor eliminates values that do not divide 48 evenly. Specifically, he crosses out options a) 20, b) 15, and c) 10 because they are not factors of 48. The remaining option d) 8 is identified as the correct answer since 8 divides 48 to yield an integer degree K of 6. This exercise reinforces the relationship between edges, vertices, and regular graph properties.
Chapters
0:00 – 1:58 00:00-01:58
The instructor introduces a graph theory problem displayed on screen asking for the possible number of vertices in a simple non-directed graph with 24 edges where every vertex has degree K. He immediately applies the Handshaking Lemma, writing '24 x 2' to represent twice the number of edges. He assigns variables |V| = x and deg(v) = K to set up the equation x * K = 48. The instructor then systematically evaluates the multiple-choice options a) 20, b) 15, c) 10, and d) 8. He crosses out options a), b), and c) because they are not divisors of 48, leaving d) 8 as the only valid solution where x * K results in an integer degree.
The video provides a concise demonstration of applying the Handshaking Lemma to determine graph properties. The central takeaway is that for a regular graph, the product of vertices and degree must equal twice the number of edges. This creates a divisibility constraint where the vertex count must be a factor of 2E. The instructor's method highlights a practical strategy for multiple-choice questions: calculate the total degree sum (2E), then check which option divides this sum evenly. This approach efficiently filters incorrect answers without needing to construct the actual graph, emphasizing algebraic verification over geometric visualization.