JEE Advanced · Mathematics · 18. Matrices

Let $M = (a_{i j}), i, j \in {1, 2, 3}$ , be the $3 \times 3$ matrix such that $a_{i j} = 1$ if $j + 1$ is divisible by $i$ , otherwise $a_{i j} = 0$ . Then which of the following statements is(are) true?

A $M$ is invertible
B There exists a nonzero column matrix $(\begin{array}{l} a_{1} \\ a_{2} \\ a_{3} \end{array})$ such that $M (\begin{array}{l} a_{1} \\ a_{2} \\ a_{3} \end{array}) = (\begin{array}{l} - a_{1} \\ - a_{2} \\ - a_{3} \end{array})$
C The set $\{X \in ℝ^{3} : M X = 0\} \neq {0}$ , where $0 = (\begin{array}{l} 0 \\ 0 \\ 0 \end{array})$
D The matrix $(M - 2 I)$ is invertible, where $I$ is the $3 \times 3$ identity matrix

Verified Solution

Answer & Solution

Correct Answer

(C) The set $\{X \in ℝ^{3} : M X = 0\} \neq {0}$ , where $0 = (\begin{array}{l} 0 \\ 0 \\ 0 \end{array})$

Step-by-step Solution

Detailed explanation

Given,

M = (a_{i j}), i, j \in {1, 2, 3}

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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Let M=aij, i, j∈{1,2,3}, be the 3×3 matrix such that aij=1 if j+1 is divisible by i, otherwise aij=0. Then which of the following statements is(are) true?

Step-by-step Solution

Let $M = (a_{i j}), i, j \in {1, 2, 3}$ , be the $3 \times 3$ matrix such that $a_{i j} = 1$ if $j + 1$ is divisible by $i$ , otherwise $a_{i j} = 0$ . Then which of the following statements is(are) true?