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If A=[30−1230041]A = \begin{bmatrix} 3 & 0 & -1 \\ 2 & 3 & 0 \\ 0 & 4 & 1 \end{bmatrix}, then which of the following is/are correct ?

A. ∣A∣=1|A| = 1

B. AA is singular matrix.

C. ∣adjA∣=1|adj A| = 1

D. ∣adj.(adjA)∣=1|adj.(adj A)| = 1

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 3

Expanding along the first row, ∣A∣=3(3−0)−0+(−1)(8−0)=9−8=1|A| = 3(3-0) - 0 + (-1)(8-0) = 9 - 8 = 1. So A is true and B is false, since ∣A∣≠0|A| \ne 0 means non-singular.

For order n=3n=3, ∣adjA∣=∣A∣n−1=12=1|adj A| = |A|^{n-1} = 1^2 = 1 and ∣adj(adjA)∣=∣A∣(n−1)2=14=1|adj(adj A)| = |A|^{(n-1)^2} = 1^4 = 1. Hence A, C and D are correct.

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