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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Determinants

NDA 2025 · Mathematics · Determinants

If vmatrix 2 & 3+i & -1 3-i & 0 & i-1 -1 & -1-i & 1 vmatrix = A +...

If ∣23+i−13−i0i−1−1−1−i1∣=A+iB\begin{vmatrix}2 & 3+i & -1 \\ 3-i & 0 & i-1 \\ -1 & -1-i & 1 \end{vmatrix} = A + iB where i=i =\sqrt{-1}, then what is A+BA + B equal to?

Options

  1. A−10-10
  2. B−6-6
  3. C00
  4. D66

Correct answer

B. −6-6

Step-by-step solution

Given the determinant:

∣2 and 3+i and −13−i and 0 and i−1−1 and −1−i and 1∣=A+iB\begin{vmatrix} 2 \text{ and } 3+i \text{ and } -1 \\ 3-i \text{ and } 0 \text{ and } i-1 \\ -1 \text{ and } -1-i \text{ and } 1 \end{vmatrix} = A + iB

Expanding the determinant along the first row, we get:

D=2∣0 and i−1−1−i and 1∣−(3+i)∣3−i and i−1−1 and 1∣−1∣3−i and 0−1 and −1−i∣D = 2 \begin{vmatrix} 0 \text{ and } i-1 \\ -1-i \text{ and } 1 \end{vmatrix} - (3+i) \begin{vmatrix} 3-i \text{ and } i-1 \\ -1 \text{ and } 1 \end{vmatrix} - 1 \begin{vmatrix} 3-i \text{ and } 0 \\ -1 \text{ and } -1-i \end{vmatrix}

Evaluating each 2×22 \times 2 minor:

First minor:
∣0 and i−1−1−i and 1∣=(0)(1)−(i−1)(−1−i)=0−(−(i2−1))=0−(−(−1−1))=−2\begin{vmatrix} 0 \text{ and } i-1 \\ -1-i \text{ and } 1 \end{vmatrix} = (0)(1) - (i-1)(-1-i) = 0 - (-(i^2 - 1)) = 0 - (-(-1 - 1)) = -2

Second minor:
∣3−i and i−1−1 and 1∣=(3−i)(1)−(i−1)(−1)=3−i+i−1=2\begin{vmatrix} 3-i \text{ and } i-1 \\ -1 \text{ and } 1 \end{vmatrix} = (3-i)(1) - (i-1)(-1) = 3 - i + i - 1 = 2

Third minor:
∣3−i and 0−1 and −1−i∣=(3−i)(−1−i)−(0)(−1)=−3−3i+i+i2=−3−2i−1=−4−2i\begin{vmatrix} 3-i \text{ and } 0 \\ -1 \text{ and } -1-i \end{vmatrix} = (3-i)(-1-i) - (0)(-1) = -3 - 3i + i + i^2 = -3 - 2i - 1 = -4 - 2i

Substituting these values back into the expansion:

D=2(−2)−(3+i)(2)−1(−4−2i)D = 2(-2) - (3+i)(2) - 1(-4 - 2i)

D=−4−(6+2i)+(4+2i)D = -4 - (6 + 2i) + (4 + 2i)

D=−4−6−2i+4+2iD = -4 - 6 - 2i + 4 + 2i

D=−6D = -6

Comparing this with A+iBA + iB, we get:
A=−6A = -6 and B=0B = 0

Therefore, A+B=−6+0=−6A + B = -6 + 0 = -6.

Answer: −6-6
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