Animated Solution for Mathematics - Vector Algebra: Consider the set of eight vectors V={ai^+bj^+ck^:a,b,c∈{−1,1}}. Three non-coplanar vectors can be chosen from V in 2p ways. Then p is
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Visualized Solution
Visualizing the Vector Set V
Given set: V={ai^+bj^+ck^:a,b,c∈{−1,1}}
Since each component a,b,c has 2 choices (1 or −1).
Total number of vectors in V=2×2×2=8.
Geometric Interpretation
These 8 vectors correspond to the position vectors of the vertices of a cube.
The cube is centered at the origin (0,0,0).
Vertices are at (±1,±1,±1).
Identifying Collinear Pairs
Notice that for every vector v, its negative −v is also in the set.
Example: (1,1,1) and (−1,−1,−1).
These pairs of opposite vectors are collinear and form the body diagonals of the cube.
The Four Body Diagonals
A cube has exactly 4 body diagonals.
Diagonal 1: (1,1,1) and (−1,−1,−1)
Diagonal 2: (1,−1,−1) and (−1,1,1)
Diagonal 3: (1,1,−1) and (−1,−1,1)
Diagonal 4: (−1,1,−1) and (1,−1,1)
Condition for Non-Coplanarity
We need to choose 3 non-coplanar vectors.
Three vectors are coplanar if they lie in the same plane.
If we choose 2 vectors from the same diagonal, they are collinear.
Any 3rd vector combined with these 2 will always form a plane, making them coplanar.
The Selection Strategy
To ensure the 3 vectors are non-coplanar, we must avoid picking 2 vectors from the same diagonal.
Therefore, the 3 vectors must be chosen from 3 distinct body diagonals.
Choosing the Diagonals
Out of the 4 available body diagonals, we need to select 3.
Number of ways to choose 3 diagonals = 4C3.
4C3=3!(4−3)!4!=4 ways.
Choosing Vectors from Diagonals
For each of the 3 chosen diagonals, we must pick exactly 1 vector.
Each diagonal has 2 vectors (one pointing in each direction).
Number of ways to choose vectors = 2×2×2=23=8 ways.
Total Combinations
Total number of ways to choose 3 non-coplanar vectors is the product of both steps.
Total ways = (Ways to choose diagonals) × (Ways to choose vectors).
Total ways = 4×8=32.
Finding p
The problem states that the number of ways is 2p.
Equating our result: 2p=32.
Since 32=25, we get p=5.
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The Sigma Insight: Scalar Triple Product
Solution Diagram
Analyzing the Setup
The set V={ai^+bj^+ck^:a,b,c∈{−1,1}} represents the eight vertices of a cube centered at the origin (0,0,0). Each component a,b,c has two possible values, leading to a total of:
2×2×2=8 distinct vectors.
Visualizing these points as vertices of a cube allows us to leverage the inherent symmetry of the structure. This geometric perspective is essential for identifying the constraints of the problem.
The Trap of Collinearity
We must select three vectors from these eight such that they are non-coplanar. A critical constraint arises from the four body diagonals of the cube. For every vector v∈V, its negative −v is also in V.
These pairs, such as (1,1,1) and (−1,−1,−1), are collinear as they lie on the same line passing through the origin. If we select two vectors from the same body diagonal, they are collinear, which forces any third vector to lie in the same plane as that line.
To ensure the three vectors are non-coplanar, we must strictly avoid picking two vectors from the same body diagonal.
The Combinatorial Strategy
To satisfy the non-coplanarity condition, we must select three vectors from three distinct body diagonals. We have four body diagonals available, and we must choose three of them. The number of ways to choose these diagonals is:
(34)=4
For each of the three chosen diagonals, we must select exactly one of the two available vectors. This provides 2 choices for each of the three diagonals, resulting in:
2×2×2=23=8 ways.
The Final Synthesis
To find the total number of successful combinations, we multiply the number of ways to choose the diagonals by the number of ways to choose the vectors within those diagonals:
Total ways=(34)×23=4×8=32
The problem states that this value is equal to 2p. Therefore, we set up the equation:
2p=32
Since 32=25, we conclude that p=5. This result demonstrates how geometric insight and combinatorial logic simplify complex spatial problems.