Sigma Percentile
JEE Advanced 2013
LEVELJEE Main

Animated Solution for Mathematics - Vector Algebra: Consider the set of eight vectors . Three non-coplanar vectors can be chosen from in ways. Then is

Enter Numerical Value:

Visualized Solution

Visualizing the Vector Set

  • Given set:
  • Since each component has choices ( or ).
  • Total number of vectors in .

Geometric Interpretation

  • These vectors correspond to the position vectors of the vertices of a cube.
  • The cube is centered at the origin .
  • Vertices are at .

Identifying Collinear Pairs

  • Notice that for every vector , its negative is also in the set.
  • Example: and .
  • These pairs of opposite vectors are collinear and form the body diagonals of the cube.

The Four Body Diagonals

  • A cube has exactly body diagonals.
  • Diagonal 1: and
  • Diagonal 2: and
  • Diagonal 3: and
  • Diagonal 4: and

Condition for Non-Coplanarity

  • We need to choose non-coplanar vectors.
  • Three vectors are coplanar if they lie in the same plane.
  • If we choose vectors from the same diagonal, they are collinear.
  • Any vector combined with these will always form a plane, making them coplanar.

The Selection Strategy

  • To ensure the vectors are non-coplanar, we must avoid picking vectors from the same diagonal.
  • Therefore, the vectors must be chosen from distinct body diagonals.

Choosing the Diagonals

  • Out of the available body diagonals, we need to select .
  • Number of ways to choose diagonals = .
  • ways.

Choosing Vectors from Diagonals

  • For each of the chosen diagonals, we must pick exactly vector.
  • Each diagonal has vectors (one pointing in each direction).
  • Number of ways to choose vectors = ways.

Total Combinations

  • Total number of ways to choose non-coplanar vectors is the product of both steps.
  • Total ways = (Ways to choose diagonals) (Ways to choose vectors).
  • Total ways = .

Finding

  • The problem states that the number of ways is .
  • Equating our result: .
  • Since , we get .

The Sigma Insight: Scalar Triple Product

Solution Diagram

Analyzing the Setup

The set represents the eight vertices of a cube centered at the origin . Each component has two possible values, leading to a total of:
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 , its negative is also in .
These pairs, such as and , 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:
For each of the three chosen diagonals, we must select exactly one of the two available vectors. This provides choices for each of the three diagonals, resulting in:

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:
The problem states that this value is equal to . Therefore, we set up the equation:
Since , we conclude that . This result demonstrates how geometric insight and combinatorial logic simplify complex spatial problems.

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