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Matrices and Determinants question

2008 · Shift 1 · Q46
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  5. /2008 · Shift 1 · Q46

Matrices and Determinants question

2008 · Shift 1 · Q46

JEE AdvancedMathematicsMatrices and DeterminantsMCQ+3 / −1
Consider the system of equations: x−2y+3z=−1−x+y−2z=kx−3y+4z=1x-2y+3z=-1-x+y-2z=kx-3y+4z=1x−2y+3z=−1−x+y−2z=kx−3y+4z=1 Statement - 1 : The system of equations has no solution for ke3k e3ke3. and Statement - 2 : The determinant ∣13−1−1−2k141∣e0\left| {\begin{matrix} 1 & 3 & { - 1} \\ { - 1} & { - 2} & k \\ 1 & 4 & 1 \\ \end{matrix} } \right| e 0​1−11​3−24​−1k1​​e0, for ke3k e 3ke3.
  1. A
    Statement - 1 is True, Statement - 2 is True; Statement - 2 is a correct explanation for Statement - 1
  2. B
    Statement - 1 is True, Statement - 2 is True; Statement - 2 is NOT a correct explanation for Statement - 1
  3. C
    Statement - 1 is True, Statement - 2 is False
  4. D
    Statement - 1 is False, Statement - 2 is True
View written solutionFree

Correct answer: A

Step-by-step Solution:

The given system of linear equations is:

  1. x−2y+3z=−1x - 2y + 3z = -1x−2y+3z=−1
  2. −x+y−2z=k-x + y - 2z = k−x+y−2z=k
  3. x−3y+4z=1x - 3y + 4z = 1x−3y+4z=1

This system can be written in matrix form as AX=BAX = BAX=B, where: A=(1−23−11−21−34)A = \begin{pmatrix} 1 & -2 & 3 \\ -1 & 1 & -2 \\ 1 & -3 & 4 \end{pmatrix}A=​1−11​−21−3​3−24​​, X=(xyz)X = \begin{pmatrix} x \\ y \\ z \end{pmatrix}X=​xyz​​, and B=(−1k1)B = \begin{pmatrix} -1 \\ k \\ 1 \end{pmatrix}B=​−1k1​​


Analysis of Statement - 1

Statement - 1 : The system of equations has no solution for k≠3k \ne 3k=3.

To determine the nature of the solution, we first calculate the determinant of the coefficient matrix, det⁡(A)\det(A)det(A).

det⁡(A)=∣1−23−11−21−34∣\det(A) = \left| {\begin{matrix} 1 & -2 & 3 \\ -1 & 1 & -2 \\ 1 & -3 & 4 \\ \end{matrix} } \right|det(A)=​1−11​−21−3​3−24​​ =1(1⋅4−(−2)(−3))−(−2)(−1⋅4−(−2)⋅1)+3(−1⋅(−3)−1⋅1)= 1(1 \cdot 4 - (-2)(-3)) - (-2)(-1 \cdot 4 - (-2) \cdot 1) + 3(-1 \cdot (-3) - 1 \cdot 1)=1(1⋅4−(−2)(−3))−(−2)(−1⋅4−(−2)⋅1)+3(−1⋅(−3)−1⋅1) =1(4−6)+2(−4+2)+3(3−1)= 1(4 - 6) + 2(-4 + 2) + 3(3 - 1)=1(4−6)+2(−4+2)+3(3−1) =1(−2)+2(−2)+3(2)= 1(-2) + 2(-2) + 3(2)=1(−2)+2(−2)+3(2) =−2−4+6=0= -2 - 4 + 6 = 0=−2−4+6=0

Since det⁡(A)=0\det(A) = 0det(A)=0, the system will have either infinitely many solutions or no solution.

To find the condition for consistency, we can use Gaussian elimination on the augmented matrix [A∣B][A|B][A∣B].

[A∣B]=[1−23∣−1−11−2∣k1−34∣1][A|B] = \left[ {\begin{matrix} 1 & -2 & 3 & | & -1 \\ -1 & 1 & -2 & | & k \\ 1 & -3 & 4 & | & 1 \\ \end{matrix} } \right][A∣B]=​1−11​−21−3​3−24​∣∣∣​−1k1​​

Apply row operations: R2→R2+R1R_2 \to R_2 + R_1R2​→R2​+R1​ R3→R3−R1R_3 \to R_3 - R_1R3​→R3​−R1​

∼[1−23∣−10−11∣k−10−11∣2]\sim \left[ {\begin{matrix} 1 & -2 & 3 & | & -1 \\ 0 & -1 & 1 & | & k-1 \\ 0 & -1 & 1 & | & 2 \\ \end{matrix} } \right]∼​100​−2−1−1​311​∣∣∣​−1k−12​​

Next, apply R3→R3−R2R_3 \to R_3 - R_2R3​→R3​−R2​:

∼[1−23∣−10−11∣k−1000∣2−(k−1)]=[1−23∣−10−11∣k−1000∣3−k]\sim \left[ {\begin{matrix} 1 & -2 & 3 & | & -1 \\ 0 & -1 & 1 & | & k-1 \\ 0 & 0 & 0 & | & 2 - (k-1) \\ \end{matrix} } \right] = \left[ {\begin{matrix} 1 & -2 & 3 & | & -1 \\ 0 & -1 & 1 & | & k-1 \\ 0 & 0 & 0 & | & 3-k \\ \end{matrix} } \right]∼​100​−2−10​310​∣∣∣​−1k−12−(k−1)​​=​100​−2−10​310​∣∣∣​−1k−13−k​​

The last row represents the equation 0⋅x+0⋅y+0⋅z=3−k0 \cdot x + 0 \cdot y + 0 \cdot z = 3 - k0⋅x+0⋅y+0⋅z=3−k, which simplifies to 0=3−k0 = 3 - k0=3−k.

  • If 3−k≠03 - k \ne 03−k=0 (i.e., k≠3k \ne 3k=3), the equation becomes 0=(non-zero value)0 = (\text{non-zero value})0=(non-zero value), which is a contradiction. In this case, the system is inconsistent and has no solution.
  • If 3−k=03 - k = 03−k=0 (i.e., k=3k = 3k=3), the equation becomes 0=00 = 00=0, which is always true. The system has infinitely many solutions.

Therefore, the system of equations has no solution for k≠3k \ne 3k=3.

Conclusion: Statement - 1 is True.


Analysis of Statement - 2

Statement - 2 : The determinant ∣13−1−1−2k141∣≠0\left| {\begin{matrix} 1 & 3 & { - 1} \\ { - 1} & { - 2} & k \\ 1 & 4 & 1 \\ \end{matrix} } \right| \ne 0​1−11​3−24​−1k1​​=0, for k≠3k \ne 3k=3.

Let's calculate the value of the determinant:

D=∣13−1−1−2k141∣D = \left| {\begin{matrix} 1 & 3 & -1 \\ -1 & -2 & k \\ 1 & 4 & 1 \\ \end{matrix} } \right|D=​1−11​3−24​−1k1​​ =1(−2⋅1−k⋅4)−3(−1⋅1−k⋅1)+(−1)(−1⋅4−(−2)⋅1)= 1(-2 \cdot 1 - k \cdot 4) - 3(-1 \cdot 1 - k \cdot 1) + (-1)(-1 \cdot 4 - (-2) \cdot 1)=1(−2⋅1−k⋅4)−3(−1⋅1−k⋅1)+(−1)(−1⋅4−(−2)⋅1) =1(−2−4k)−3(−1−k)−1(−4+2)= 1(-2 - 4k) - 3(-1 - k) - 1(-4 + 2)=1(−2−4k)−3(−1−k)−1(−4+2) =−2−4k+3+3k−1(−2)= -2 - 4k + 3 + 3k - 1(-2)=−2−4k+3+3k−1(−2) =−2−4k+3+3k+2= -2 - 4k + 3 + 3k + 2=−2−4k+3+3k+2 =(−2+3+2)+(−4k+3k)= (-2 + 3 + 2) + (-4k + 3k)=(−2+3+2)+(−4k+3k) =3−k= 3 - k=3−k

The statement says that this determinant is not equal to zero for k≠3k \ne 3k=3. If D=0D = 0D=0, then 3−k=03 - k = 03−k=0, which means k=3k = 3k=3. So, the determinant is zero only when k=3k=3k=3. For any value k≠3k \ne 3k=3, the determinant D=3−kD = 3-kD=3−k will be non-zero.

Conclusion: Statement - 2 is True.


Relationship between Statement - 1 and Statement - 2

  • We found that Statement 1 is true. The condition for the system to have no solution is k≠3k \ne 3k=3. The mathematical reason is that det⁡(A)=0\det(A) = 0det(A)=0 and the system becomes inconsistent if 3−k≠03-k \ne 03−k=0.

  • We found that Statement 2 is also true. It states that a certain determinant is non-zero for k≠3k \ne 3k=3. This determinant evaluates to 3−k3-k3−k. So, Statement 2 is equivalent to the condition 3−k≠03-k \ne 03−k=0 for k≠3k \ne 3k=3.

  • The condition 3−k≠03-k \ne 03−k=0 is precisely the reason why the system in Statement 1 is inconsistent (has no solution), given that we already established det⁡(A)=0\det(A)=0det(A)=0. Statement 2 provides this crucial condition.

Therefore, Statement 2 is a correct explanation for Statement 1.

Final Choice: Statement - 1 is True, Statement - 2 is True; Statement - 2 is a correct explanation for Statement - 1. This corresponds to option A.

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