JEE Challenger
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Consistency of Linear System and Determinant Condition

Consider the system of equations

x−2y+3z=−1−x+y−2z=kx−3y+4z=1\begin{array}{l} x - 2y + 3z = -1 \\ -x + y - 2z = k \\ x - 3y + 4z = 1 \end{array}

STATEMENT-1 : The system of equations has no solution for k≠3k \neq 3 .
and
STATEMENT-2 : The determinant ∣13−1−1−2k141∣≠0\begin{vmatrix} 1 & 3 & -1 \\ -1 & -2 & k \\ 1 & 4 & 1 \end{vmatrix} \neq 0, for k≠3k \neq 3 .

Options

A

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is a correct explanation for STATEMENT-1

Correct
B

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is NOT a correct explanation for STATEMENT-1

C

STATEMENT-1 is True, STATEMENT-2 is False

D

STATEMENT-1 is False, STATEMENT-2 is True

Step-by-Step Solution

To determine the consistency of the system of linear equations, we write the system in matrix form:

x−2y+3z=−1−x+y−2z=kx−3y+4z=1\begin{array}{rcccl} x &-& 2y &+& 3z = -1 \\ -x &+& y &-& 2z = k \\ x &-& 3y &+& 4z = 1 \end{array}

First, let us evaluate the determinant of the coefficient matrix, DD:

D=∣1−23−11−21−34∣D = \begin{vmatrix} 1 & -2 & 3 \\ -1 & 1 & -2 \\ 1 & -3 & 4 \end{vmatrix}

Applying the elementary row operations R2→R2+R1R_2 \to R_2 + R_1 and R3→R3−R1R_3 \to R_3 - R_1:

D=∣1−230−110−11∣=1((−1)(1)−(1)(−1))=0D = \begin{vmatrix} 1 & -2 & 3 \\ 0 & -1 & 1 \\ 0 & -1 & 1 \end{vmatrix} = 1((-1)(1) - (1)(-1)) = 0

Since D=0D = 0, by Cramer's Rule, the system of equations will have:

  1. Infinitely many solutions if Dx=Dy=Dz=0D_x = D_y = D_z = 0.
  2. No solution (inconsistent) if at least one of Dx,Dy,Dz≠0D_x, D_y, D_z \neq 0.

Now, let us evaluate the determinant given in STATEMENT-2:

Δ=∣13−1−1−2k141∣\Delta = \begin{vmatrix} 1 & 3 & -1 \\ -1 & -2 & k \\ 1 & 4 & 1 \end{vmatrix}

Expanding along the first row:
Δ=1((−2)(1)−(k)(4))−3((−1)(1)−(k)(1))−1((−1)(4)−(−2)(1))\Delta = 1((-2)(1) - (k)(4)) - 3((-1)(1) - (k)(1)) - 1((-1)(4) - (-2)(1))
Δ=1(−2−4k)−3(−1−k)−1(−4+2)\Delta = 1(-2 - 4k) - 3(-1 - k) - 1(-4 + 2)
Δ=−2−4k+3+3k+2=3−k\Delta = -2 - 4k + 3 + 3k + 2 = 3 - k

Thus, Δ≠0\Delta \neq 0 for k≠3k \neq 3.
Hence, STATEMENT-2 is True.

Notice that the determinant Δ\Delta is related to DyD_y (the determinant obtained by replacing the column of coefficients of yy with the column of constants):

Dy=∣1−13−1k−2114∣D_y = \begin{vmatrix} 1 & -1 & 3 \\ -1 & k & -2 \\ 1 & 1 & 4 \end{vmatrix}

Interchanging Column 2 and Column 3 yields:

Dy=−∣13−1−1−2k141∣=−Δ=k−3D_y = - \begin{vmatrix} 1 & 3 & -1 \\ -1 & -2 & k \\ 1 & 4 & 1 \end{vmatrix} = -\Delta = k - 3

Since D=0D = 0 and Dy=k−3≠0D_y = k - 3 \neq 0 for k≠3k \neq 3, at least one of the determinants Dx,Dy,DzD_x, D_y, D_z is non-zero.
Therefore, the given system of linear equations is inconsistent and has no solution for k≠3k \neq 3.

Hence, STATEMENT-1 is True, and STATEMENT-2 is the correct explanation for STATEMENT-1.

Therefore, the correct option is (A).