8.5 Practice Question
Duration: 2 min
This video lesson is available to enrolled students.
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This video segment presents a practice problem applying the Handshaking Lemma in graph theory. The instructor works through a specific question involving a simple graph with 35 edges and vertices of varying degrees. The core concept demonstrated is that the sum of all vertex degrees in a graph must equal twice the number of edges. The problem asks to find the unknown number of vertices with degree 2, given specific counts for other degrees.
Chapters
0:00 – 1:58 00:00-01:58
The instructor solves a graph theory problem using the Handshaking Lemma. Visible on-screen text states: 'Q Consider a simple graph with 35 edges such that 4 vertex of degree 5, 5 vertex of degree 4, 4 vertex of degree 3,…'. The instructor sets up the equation '4 x 5 + 5 x 4 + 4 x 3 + x x 2 = 35 x 2'. Key visible events include correcting the edge count to 35 and writing '4 x 5 + 5 x 4 + 41' initially before simplifying to '20 + 20 + 12 + 2x = 70'. The final calculation shows '2x = 70 - 52' leading to the circled answer 'x = 9'. The instructor applies the rule Sum of degrees = 2 * |E| to isolate x.
The instructional content focuses on the practical application of the Handshaking Lemma to solve for unknown vertex counts. The problem requires translating a word problem into an algebraic equation where the sum of products (vertex count * degree) equals twice the edge count. The video demonstrates the step-by-step simplification from '4 x 5 + 5 x 4 + 4 x 3 + x x 2 = 70' to '52 + 2x = 70', then solving for x. This reinforces the fundamental graph theory principle that every edge contributes exactly two to the total degree sum.