The trace of a square matrix is defined to be the sum of its diagonal entries. If A is a 2×2 matrix such that the trace of A is 3 and the trace of A3 is -18 then the value of the determinant of A is_____

JEE Advanced · Mathematics · 18. Matrices

The trace of a square matrix is defined to be the sum of its diagonal entries. If $A$ is a $2 \times 2$ matrix such that the trace of $A$ is $3$ and the trace of $A^{3}$ is $- 18$ then the value of the determinant of $A$ is_____

A 5
B 11
C 19
D 21

Verified Solution

Answer & Solution

Correct Answer

(A) 5

Step-by-step Solution

Detailed explanation

Let

A = [\begin{matrix} a & b \\ c & 3 - a \end{matrix}] \Rightarrow |A| = 3 a - a^{2} - b c

A^{3} = [\begin{matrix} a & b \\ c & 3 - a \end{matrix}] [\begin{matrix} a & b \\ c & 3 - a \end{matrix}] [\begin{matrix} a & b \\ c & 3 - a \end{matrix}]

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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