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For a square matrix A=[2130−14120]A = \begin{bmatrix} 2 & 1 & 3 \\ 0 & -1 & 4 \\ 1 & 2 & 0 \end{bmatrix}, which of the following statements is/are TRUE ?

A. det (A) = 9.

B. A21.A33=−12A_{21}.A_{33} = -12, where AijA_{ij} is the cofactor of aija_{ij}

C. det (A) = a11A11+a12A12+a13A31a_{11}A_{11} + a_{12}A_{12} + a_{13}A_{31}.

D. ∣AA′∣≠∣A∣2\lvert AA'\rvert \neq \lvert A\rvert^2

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 3

Expanding, det⁡A=−16+4+3=−9\det A=-16+4+3=-9, so A is false.

A21=6A_{21}=6 and A33=−2A_{33}=-2, so A21A33=−12A_{21}A_{33}=-12; B is true.

C mixes cofactors from different rows and gives 9≠−99\neq -9, so it is false.

∣AA′∣=∣A∣∣A′∣=∣A∣2|AA'|=|A||A'|=|A|^2 always, so D is false. Only B holds.

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