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

Defining the Vector Set

  • Set
  • Total number of vectors in .
  • These 8 vectors represent the vertices of a cube centered at .

Identifying Opposite Pairs

  • The 8 vectors form 4 pairs of opposite vectors: .
  • Pair 1: and
  • Pair 2: and
  • Pair 3: and
  • Pair 4: and

Collinearity of Opposite Vectors

  • Consider Pair 1: and .
  • Since , they are collinear.
  • They lie on the same line passing through the origin.

The Coplanarity Trap

  • If we select an opposite pair , they form a line.
  • Any third vector chosen from the set will form a plane with this line.
  • Thus, the triplet will always be coplanar.

Condition for Non-Coplanarity

  • To ensure the 3 chosen vectors are non-coplanar, we must avoid picking any opposite pair.
  • Constraint: The 3 vectors must be chosen from 3 different pairs.

Step 1: Selecting the Pairs

  • We have a total of 4 opposite pairs.
  • We need to choose 3 pairs out of these 4.
  • Number of ways to choose pairs .

Evaluating Pair Selection

  • ways.

Step 2: Selecting Vectors from Pairs

  • From each of the 3 chosen pairs, we must pick exactly 1 vector.
  • Each pair contains 2 vectors.
  • Number of ways .

Evaluating Vector Selection

  • Ways to choose vectors
  • ways.

Total Number of Ways

  • Total ways
  • Total ways
  • Total ways .

Finding the Value of

  • The problem states the total number of ways is .
  • We found the total ways to be .
  • Equating them: .
  • Since , we get .

The Sigma Insight: Scalar Triple Product

Solution Diagram

Analyzing the Geometric Setup

The set represents the eight vertices of a cube centered at the origin. Since each component has two possible values, we confirm the total number of vectors is .
These vectors exhibit central symmetry. For every vector , its diametrically opposite vector is also in . This partitions the set of eight vectors into four pairs of opposite vectors.

Identifying the Constraint

We are looking for the number of ways to choose three vectors from such that they are not coplanar. A set of three vectors is coplanar if they lie in the same plane passing through the origin.
If we select both vectors from any opposite pair, they are collinear. Any third vector chosen will then form a plane with this line, resulting in a coplanar triplet. Therefore, to ensure the three vectors are non-coplanar, we must select three vectors such that no two are opposite to each other.

The Combinatorial Calculation

To satisfy the condition, we must choose three distinct pairs out of the four available pairs. The number of ways to select these pairs is given by the combination formula:
Once we have selected three pairs, we must choose exactly one vector from each of the three chosen pairs. Since each pair contains two vectors, there are choices for each pair.
For a specific selection of three pairs, the number of ways to pick the vectors is:

Final Calculation

To find the total number of non-coplanar triplets, we multiply the number of ways to choose the pairs by the number of ways to choose the vectors within those pairs:
Given that the problem states the answer is in the form , we set:
Thus, we find that .

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