Question:medium

If $A = \begin{bmatrix} 1 & 2 & 4 \\ -1 & 3 & 0 \\ 1 & 2 & 0 \end{bmatrix}$, then the value of determinant of A is:}

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Whenever a determinant contains several zero entries, always expand along the row or column having the greatest number of zeros. This minimizes calculations and significantly reduces the possibility of arithmetic mistakes.
  • $-10$
  • $10$
  • $20$
  • $-20$
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The Correct Option is D

Solution and Explanation


Step 1: Expand the determinant along the third column.
The sign pattern for cofactors in a $3 \times 3$ determinant is: \[ \begin{bmatrix} + & - & + - & + & - + & - & + \end{bmatrix} \] The elements of the third column are: \[ 4,\quad 0,\quad 0 \] Hence, \[ |A| = 4 \begin{vmatrix} -1 & 3 1 & 2 \end{vmatrix} -0 \begin{vmatrix} 1 & 2 1 & 2 \end{vmatrix} +0 \begin{vmatrix} 1 & 2 -1 & 3 \end{vmatrix} \] Since the last two terms are multiplied by zero, they vanish completely. Therefore, \[ |A| = 4 \begin{vmatrix} -1 & 3 1 & 2 \end{vmatrix} \]

Step 2: Evaluate the remaining $2 \times 2$ determinant.
For a matrix \[ \begin{bmatrix} a & b c & d \end{bmatrix}, \] the determinant is given by \[ ad-bc. \] Applying this formula: \[ \begin{vmatrix} -1 & 3 1 & 2 \end{vmatrix} = (-1)(B)-(C)(A) \] \[ =-2-3 \] \[ =-5 \] Thus, \[ \begin{vmatrix} -1 & 3 1 & 2 \end{vmatrix} =-5. \]

Step 3: Substitute the value into the expansion.
Using the result obtained above, \[ |A| = 4(-5) \] \[ |A| = -20 \]

Step 4: Verify the result.
Since only one non-zero cofactor term contributed to the expansion, the computation is straightforward and free from sign ambiguity. Thus, the determinant of matrix $A$ is \[ {-20} \] Hence, the correct answer is: \[ {(D)\ -20} \]
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