Medium MCQ +4 / -1 PYQ · JEE Mains 2021

Let A and B be 3 $\times$ 3 real matrices such that A is symmetric matrix and B is skew-symmetric matrix. Then the system of linear equations (A2B2 $-$ B2A2) X = O, where X is a 3 $\times$ 1 column matrix of unknown variables and O is a 3 $\times$ 1 null matrix, has :

  1. A no solution
  2. B exactly two solutions
  3. C infinitely many solutions Correct answer
  4. D a unique solution

Solution

A<sup>T</sup> = A, B<sup>T</sup> = $-$B<br><br>Let A<sup>2</sup>B<sup>2</sup> $-$ B<sup>2</sup>A<sup>2</sup> = P<br><br>P<sup>T</sup> = (A<sup>2</sup>B<sup>2</sup> $-$ B<sup>2</sup>A<sup>2</sup>)<sup>T</sup> = (A<sup>2</sup>B<sup>2</sup>)<sup>T</sup> $-$ (B<sup>2</sup>A<sup>2</sup>)<sup>T</sup><br><br>= (B<sup>2</sup>)<sup>T</sup> (A<sup>2</sup>)<sup>T</sup> $-$ (A<sup>2</sup>)<sup>T</sup> (B<sup>2</sup>)<sup>T</sup><br><br>= B<sup>2</sup>A<sup>2</sup> $-$ A<sup>2</sup>B<sup>2</sup><br><br>$\Rightarrow$ P is skew-symmetric matrix<br><br>$$\left[ {\matrix{ 0 &amp; a &amp; b \cr { - a} &amp; 0 &amp; c \cr { - b} &amp; { - c} &amp; 0 \cr } } \right]\left[ {\matrix{ x \cr y \cr z \cr } } \right] = \left[ {\matrix{ 0 \cr 0 \cr 0 \cr } } \right]$$<br><br>$\therefore$ ay + bz = 0 ..... (1)<br><br>$-$ax + cz = 0 .... (2)<br><br>$-$bx $-$cy = 0 ..... (3)<br><br>From equation 1, 2, 3<br><br>$\Delta$ = 0 &amp; $\Delta$<sub>1</sub> = $\Delta$<sub>2</sub> = $\Delta$<sub>3</sub> = 0<br><br>$\therefore$ equation have infinite number of solution

About this question

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

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