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If AA and BB are invertible matrices of order 33 then match List-I with List-II

List-IList-II
(A) adj(A)\text{adj}(A)(I) B−1A−1B^{-1} A^{-1}
(B) (AB)−1(AB)^{-1}(II) ∣A∣−1\vert A\vert ^{-1}
(C) ∣A−1∣\vert A^{-1}\vert (III) ∣A∣2\vert A\vert ^2
(D) ∣adjA∣\vert \text{adj} A\vert (IV) ∣A∣A−1\vert A\vert A^{-1}

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 3

For an invertible matrix AA of order nn:

A−1=adj(A)∣A∣A^{-1} = \dfrac{\text{adj}(A)}{|A|}, i.e., adj(A)=∣A∣⋅A−1\text{adj}(A) = |A| \cdot A^{-1}

(AB)−1=B−1A−1(AB)^{-1} = B^{-1}A^{-1}

∣A−1∣=∣A∣−1|A^{-1}| = |A|^{-1}

∣adj(A)∣=∣A∣n−1|\text{adj}(A)| = |A|^{n-1}


adj(A)\text{adj}(A)

From the inverse formula: A−1=adj(A)∣A∣A^{-1} = \dfrac{\text{adj}(A)}{|A|}

adj(A)=∣A∣⋅A−1\text{adj}(A) = |A| \cdot A^{-1}

(A)⟶(IV)(A) \longrightarrow (IV)


(AB)−1(AB)^{-1}

(AB)−1=B−1A−1(AB)^{-1} = B^{-1}A^{-1}

(B)⟶(I)(B) \longrightarrow (I)


∣A−1∣|A^{-1}|

Taking determinant on both sides of A⋅A−1=IA \cdot A^{-1} = I:

∣A∣⋅∣A−1∣=∣I∣=1|A| \cdot |A^{-1}| = |I| = 1

∣A−1∣=1∣A∣|A^{-1}| = \dfrac{1}{|A|}

=∣A∣−1= |A|^{-1}

(C)⟶(II)(C) \longrightarrow (II)


∣adj(A)∣|\text{adj}(A)|

Since adj(A)=∣A∣⋅A−1\text{adj}(A) = |A| \cdot A^{-1}, taking determinant of both sides with n=3n = 3:

∣adj(A)∣=∣∣A∣⋅A−1∣|\text{adj}(A)| = \big||A| \cdot A^{-1}\big|

=∣A∣3⋅∣A−1∣= |A|^3 \cdot |A^{-1}|

=∣A∣3⋅1∣A∣= |A|^3 \cdot \dfrac{1}{|A|}

=∣A∣2= |A|^2

(D)⟶(III)(D) \longrightarrow (III)


(A)→(IV)(A) \rightarrow (IV), (B)→(I)(B) \rightarrow (I), (C)→(II)(C) \rightarrow (II), (D)→(III)(D) \rightarrow (III)

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