Permutations & Combinations
Geometric Counting
Grade 11

Question:

<p>How many line segments have both their endpoints located at the vertices of a given cube.</p>
<p>(A) 36</p>
<p>(B) 28</p>
<p>(C) 45</p>
<p>(D) 21</p>

Step-by-Step Solution

Key Concept: Count all possible line segments between 8 vertices using combinations, which automatically includes edges, face diagonals, and space diagonals.
<p><strong>Solution:</strong> A cube has 8 vertices. Total line segments connecting any two vertices = \(\binom{8}{2} = 28\)</p><p>These 28 segments consist of:</p><p>1. Edges of the cube: 12</p><p>2. Face diagonals: 12 (2 diagonals per face × 6 faces)</p><p>3. Space diagonals: 4 (connecting opposite vertices)</p><p>Total = 12 + 12 + 4 = 28</p><p>∴ Answer is (B) 28</p>
Correct Answer: B

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