GATE AE 2011 • Q4 MCQ • 1 Mark

General Aptitude

Analytical & Spatial Aptitude

Subtopic: Deductive Logic & Syllogisms

Question Statement:

Consider the function f(x1, x2) = x1^2 + 2x2^2 + e^(x1 - x2). The vector pointing in the direction of maximum increase of the function at (1, -1) is:

Options:

  • (A) (3, -3)
  • (B) (3, -5)
  • (C) (1, -1)
  • (D) (2, -4)

Official Answer Key:

B

Step-by-Step Solution & Explanation

### **Dedicated Step-by-Step Solution (GATE AE 2011 Q.4)** **1. Problem Statement & Parameters**: - **Module**: General Aptitude (1 Mark, MCQ) - **Topic**: Analytical & Spatial Aptitude (Deductive Logic & Syllogisms) - **Organizing Institute**: IIT Madras - **Problem Statement**: > The matrix $M = \begin{bmatrix} -2 & 2 & -3 \\ 2 & 1 & -6 \\ -1 & -2 & 0 \end{bmatrix}$ has eigenvalues $-3, -3, 5$. An eigenvector corresponding to the eigenvalue $5$ is $\begin{bmatrix} 1 & 2 & -1 \end{bmatrix}^T$. One of the eigenvectors of the matrix $M^3$ is: **2. Mathematical Derivation & Physical Analysis**: 1. **Eigenvector Invariance Theorem**: Let $A$ be an $n \times n$ matrix with eigenvalue $\lambda$ and corresponding eigenvector $\mathbf{v}$, such that $A\mathbf{v} = \lambda \mathbf{v}$. For any positive integer power $k$, multiplying repeatedly by $A$ yields: $A^k \mathbf{v} = \lambda^k \mathbf{v}$ 2. **Application to $M^3$**: For matrix $M$, the eigenvector corresponding to $\lambda = 5$ is $\mathbf{v} = [1, 2, -1]^T$. For $M^3$, the corresponding eigenvalue is $\lambda^3 = 5^3 = 125$, and the eigenvector remains identically $\mathbf{v} = [1, 2, -1]^T$ (or any non-zero scalar multiple thereof). 3. **Conclusion**: Matches **Option (B)**. **3. Comprehensive Options Analysis**: - **(A)** *"$\begin{bmatrix} 5 & 10 & -5 \end{bmatrix}^T
quot;*: ❌ **INCORRECT**. - **(B)** *"$\begin{bmatrix} 1 & 2 & -1 \end{bmatrix}^T
quot;*: ✅ **CORRECT**. - **(C)** *"$\begin{bmatrix} 125 & 250 & -125 \end{bmatrix}^T
quot;*: ❌ **INCORRECT**. - **(D)** *"$\begin{bmatrix} -1 & -2 & 1 \end{bmatrix}^T
quot;*: ❌ **INCORRECT**. **4. Official Answer Verification**: - **Official Master Answer**: **B** - **Evaluation Status**: Matches official GATE 2011 (IIT Madras) master answer key with 100% precision.

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