Medium MCQ +4 / -1 PYQ · JEE Mains 2022

Let $\mathrm{A}$ and $\mathrm{B}$ be any two $3 \times 3$ symmetric and skew symmetric matrices respectively. Then which of the following is NOT true?

  1. A $\mathrm{A}^{4}-\mathrm{B}^{4}$ is a smmetric matrix
  2. B $\mathrm{AB}-\mathrm{BA}$ is a symmetric matrix
  3. C $\mathrm{B}^{5}-\mathrm{A}^{5}$ is a skew-symmetric matrix Correct answer
  4. D $\mathrm{AB}+\mathrm{BA}$ is a skew-symmetric matrix

Solution

<p>(A) $M = {A^4} - {B^4}$</p> <p>${M^T} = {({A^4} - {B^4})^T} = {({A^T})^4} - {({B^T})^4}$</p> <p>$= {A^4} - {( - B)^4} = {A^4} - {B^4} = M$</p> <p>(B) $M = AB - BA$</p> <p>${M^T} = {(AB - BA)^T} = {(AB)^T} - {(BA)^T}$</p> <p>$= {B^T}{A^T} - {A^T}{B^T}$</p> <p>$= - BA - A( - B)$</p> <p>$= AB - BA = M$</p> <p>(C) $M = {B^5} - {A^5}$</p> <p>${M^T} = {({B^T})^5} - {({A^T})^5} = - ({B^5} + {A^5}) \ne - M$</p> <p>(D) $M = AB + BA$</p> <p>${M^T} = {(AB)^T} + {(BA)^T}$</p> <p>$= {B^T}{A^T} + {A^T}{B^T} = - BA - AB = - M$</p>

About this question

Subject: Mathematics · Chapter: Matrices and Determinants · Topic: Types of Matrices and Operations

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