NIMCET 2024 — Mathematics PYQ
NIMCET | Mathematics | 2024For an invertible matrix A, which of the following is not always true:
Choose the correct answer:
- A.
∣adj(A)∣=0
- B.
∣A∣=0
- C.
∣AA−1∣=1
∣A⋅adj(A)∣=1
Explanation
Solution:
Consider the property: A⋅adj(A)=∣A∣I
Taking determinant on both sides:
∣A⋅adj(A)∣=∣∣A∣I∣
For a matrix of order n:
∣∣A∣I∣=∣A∣n∣I∣=∣A∣n
Thus, ∣A⋅adj(A)∣=∣A∣n
If ∣A∣=1, then ∣A⋅adj(A)∣=1n=1.
In this specific case, the statement ∣A⋅adj(A)∣=1 becomes false.
Since an invertible matrix can have a determinant equal to 1 (e.g., identity matrix), the condition ∣A⋅adj(A)∣=1 is not always true.
Explanation
Solution:
Consider the property: A⋅adj(A)=∣A∣I
Taking determinant on both sides:
∣A⋅adj(A)∣=∣∣A∣I∣
For a matrix of order n:
∣∣A∣I∣=∣A∣n∣I∣=∣A∣n
Thus, ∣A⋅adj(A)∣=∣A∣n
If ∣A∣=1, then ∣A⋅adj(A)∣=1n=1.
In this specific case, the statement ∣A⋅adj(A)∣=1 becomes false.
Since an invertible matrix can have a determinant equal to 1 (e.g., identity matrix), the condition ∣A⋅adj(A)∣=1 is not always true.

