Question:medium

If \(A = \left[ \begin{array}{ccc}0 & 0 & -1 \\ 0 & -1 & 0 \\ -1 & 0 & 0\end{array} \right]\) then which of the following is true ?

Show Hint

Square the matrix. If A times A gives the identity, A is its own inverse.
Updated On: Oct 1, 2026
  • \(A^2 = A^{-1}\)
  • \(A = -(\text{adj}A)\)
  • \(A^{-1} = A\)
  • \(A^{-1} = -A\)
Show Solution

The Correct Option is C

Solution and Explanation

Step 1: Set up:
The matrix $A$ has entries $-1$ at positions $(1,3),(2,2),(3,1)$ and zeros elsewhere. Each row has a single nonzero entry, so products are easy.

Step 2: Treat it as a signed permutation:
Write $A = -P$, where $P$ swaps the first and third coordinates and keeps the second. A swap applied twice gives back the identity, so $P^2 = I$. Then $A^2 = (-P)^2 = P^2 = I$.

Step 3: Inverse:
From $A^2 = I$ we get $A^{-1} = A$. Option (C) is correct.

Step 4: Test the distractors with the determinant:
$|A| = |-P| = (-1)^3 |P| = (-1)^3(-1) = 1$, because $P$ is one swap and has determinant $-1$. So $\text{adj}A = |A|A^{-1} = A$. Option (B) says $A = -\text{adj}A$, which would force $A = -A$, so it is false. Options (A) and (D) fail since $A^2 = I \neq A$ and $-A \neq A$.

Final Answer:
Since $A^2=I$, $A^{-1}=A$. \[ \boxed{A^{-1}=A} \]
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