Sigma Percentile
JEE Advanced 1994
LEVELJEE Main

Animated Solution for Mathematics - Vector Algebra: Let be distinct real numbers. The points with position vectors , ,

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Visualized Solution

Visualizing the Points in Space

  • Let the three given points be , , and .
  • These points are formed by cyclic permutations of the coordinates .
  • We want to determine the geometric nature of the triangle formed by these three points.

The Distance Formula

  • To find the nature of the triangle, we must calculate the lengths of its sides: , , and .
  • Recall the distance formula between two points and in space:

Setting up Distance

  • Let's substitute the coordinates of and into the distance formula.
  • This represents the distance between the first two vertices.

Simplifying the Expression for

  • Since , we can rewrite the terms to keep them in a standard cyclic order:
  • This is a symmetric expression involving the squared differences of all three coordinates.

Setting up Distance

  • Now, let's find the length of the second side, , using the coordinates and .

Comparing with

  • Let's rearrange the terms inside the square root of to match the order in :
  • Notice that the expression is mathematically identical to . Therefore, .

Setting up Distance

  • Finally, let's calculate the length of the third side, , using the coordinates and .

Comparing with and

  • Rearranging the terms for once again gives:
  • This is exactly the same expression as and . Thus, .

Establishing the Equilateral Nature

  • Since all three side lengths are equal:
  • The triangle formed by these three points must be an equilateral triangle.
  • This elegant symmetry arises because the coordinates are cyclic permutations of each other.

The Sigma Insight: Components of a Vector

Analyzing the Setup

Imagine you are standing in a vast coordinate system. You are given three points, , , and .
At first glance, they might look like a random collection of variables. However, each point is simply a cyclic shift of the previous one. This is the hallmark of a highly symmetric system, and in geometry, symmetry is almost always a shortcut to elegance.

The Tool of Choice

To understand the nature of the triangle formed by these points, we need to determine the lengths of its sides. We reach for our most trusted tool in geometry: the distance formula.
For any two points and , the distance is given by:
This formula serves as our bridge between algebra and the physical shape of the triangle.

The Calculation

Let us calculate the length of side . Substituting our coordinates, we get:
Now, let us look at side . Using the coordinates of and , we find:
Notice that the terms inside the square root are identical to those in , just in a different order. Because addition is commutative, is exactly equal to .
Finally, we calculate :
Again, the terms are identical to the previous calculations.

The Conclusion

We have proven that . A triangle where all three sides are equal is, by definition, an equilateral triangle.
This result is not just a calculation; it is a testament to the power of symmetry. Even when the variables are unknown, the cyclic structure forces the points to maintain a perfect, balanced relationship.
You have just navigated a complex problem by simply observing the underlying pattern. Keep this intuition sharp—it is exactly what you need to excel in JEE Advanced.

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