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If AA is a 3×33 \times 3 matrix such that ∣adjA∣=9|adj A| = 9 and ∣kA−1∣=9|kA^{-1}| = 9, then the value of kk are:

Solution

✅ Correct Option: 3

Given AA is a 3×33 \times 3 matrix with ∣adjA∣=9|adj A| = 9 and ∣kA−1∣=9|kA^{-1}| = 9.

For any n×nn \times n matrix, ∣adjA∣=∣A∣n−1|adj A| = |A|^{n-1}.

Since AA is a 3×33 \times 3 matrix:

∣adjA∣=∣A∣3−1|adj A| = |A|^{3-1}

∣adjA∣=∣A∣2|adj A| = |A|^2

Substituting the given value:

∣A∣2=9|A|^2 = 9

∣A∣=±3|A| = \pm 3


For a scalar kk and n×nn \times n matrix BB, ∣kB∣=kn∣B∣|kB| = k^n|B|.

Since AA is a 3×33 \times 3 matrix:

∣kA−1∣=k3∣A−1∣|kA^{-1}| = k^3|A^{-1}|

For inverse matrices, ∣A−1∣=1∣A∣|A^{-1}| = \frac{1}{|A|}.

Therefore:

∣kA−1∣=k3⋅1∣A∣|kA^{-1}| = k^3 \cdot \frac{1}{|A|}

Substituting the given value:

k3⋅1∣A∣=9k^3 \cdot \frac{1}{|A|} = 9

k3=9∣A∣k^3 = 9|A|


When ∣A∣=3|A| = 3:

k3=9×3k^3 = 9 \times 3

k3=27k^3 = 27

k=3k = 3


When ∣A∣=−3|A| = -3:

k3=9×(−3)k^3 = 9 \times (-3)

k3=−27k^3 = -27

k=−3k = -3


Therefore, the value of k=±3k = \pm 3.

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