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JEE Main 2022
Matrices & Determinants
Matrices and Determinants
Hard

Question

Let A be a 3 ×\times 3 real matrix such that A(\matrix1\cr1\cr0\cr)=(\matrix1\cr1\cr0\cr);A(\matrix1\cr0\cr1\cr)=(\matrix1\cr0\cr1\cr)A\left( {\matrix{ 1 \cr 1 \cr 0 \cr } } \right) = \left( {\matrix{ 1 \cr 1 \cr 0 \cr } } \right);A\left( {\matrix{ 1 \cr 0 \cr 1 \cr } } \right) = \left( {\matrix{ { - 1} \cr 0 \cr 1 \cr } } \right) and A(\matrix0\cr0\cr1\cr)=(\matrix1\cr1\cr2\cr)A\left( {\matrix{ 0 \cr 0 \cr 1 \cr } } \right) = \left( {\matrix{ 1 \cr 1 \cr 2 \cr } } \right). If X=(x1,x2,x3)TX = {({x_1},{x_2},{x_3})^T} and I is an identity matrix of order 3, then the system (A2I)X=(\matrix4\cr1\cr1\cr)(A - 2I)X = \left( {\matrix{ 4 \cr 1 \cr 1 \cr } } \right) has :

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Solution

This problem requires us to determine the nature of solutions for a system of linear equations (A2I)X=B(A - 2I)X = B. The core concept lies in analyzing the determinant and rank of the coefficient matrix (A2I)(A - 2I).

Key Concepts for Systems of Linear Equations CX=BCX = B:

For a system of linear equations CX=BCX = B, where CC is a square matrix of order nn:

  1. Unique Solution: If det(C)0\det(C) \neq 0, then the matrix CC is invertible, and a unique solution X=C1BX = C^{-1}B exists.
  2. No Solution or Infinitely Many Solutions: If det(C)=0\det(C) = 0, then the matrix CC is singular. In this case, we need to check the consistency of the system using the concept of rank:
    • If rank(C)=rank([CB])<n\text{rank}(C) = \text{rank}([C|B]) < n (where [CB][C|B] is the augmented matrix), the system is consistent and has infinitely many solutions. This occurs when the right-hand side vector BB can be expressed as a linear combination of the column vectors of CC, and there are free variables.
    • If rank(C)rank([CB])\text{rank}(C) \neq \text{rank}([C|B]), the system is inconsistent and has no solution. This occurs when the right-hand side vector BB cannot be expressed as a linear combination of the column vectors of CC.

Our strategy will be:

  1. First, determine the matrix AA using the given conditions.
  2. Then, calculate the coefficient matrix C=(A2I)C = (A - 2I).
  3. Calculate det(C)\det(C).
  4. If det(C)=0\det(C) = 0, we will analyze the system (A2I)X=B(A - 2I)X = B directly to check for consistency and determine if there are no solutions or infinitely many solutions.

Step 1: Determining the Matrix A

Let the 3x3 matrix AA be represented as: A=(abcdefghi)A = \begin{pmatrix} a & b & c \\ d & e & f \\ g & h & i \end{pmatrix}

We are given three conditions involving matrix AA and specific column vectors:

Condition 1: A(110)=(110)A \begin{pmatrix} 1 \\ 1 \\ 0 \end{pmatrix} = \begin{pmatrix} 1 \\ 1 \\ 0 \end{pmatrix} Performing the matrix multiplication: (abcdefghi)(110)=(a(1)+b(1)+c(0)d(1)+e(1)+f(0)g(1)+h(1)+i(0))=(a+bd+eg+h)\begin{pmatrix} a & b & c \\ d & e & f \\ g & h & i \end{pmatrix} \begin{pmatrix} 1 \\ 1 \\ 0 \end{pmatrix} = \begin{pmatrix} a(1) + b(1) + c(0) \\ d(1) + e(1) + f(0) \\ g(1) + h(1) + i(0) \end{pmatrix} = \begin{pmatrix} a+b \\ d+e \\ g+h \end{pmatrix} Equating this to the given result: (a+bd+eg+h)=(110){a+b=1(Eq. 1.1)d+e=1(Eq. 1.2)g+h=0(Eq. 1.3)\begin{pmatrix} a+b \\ d+e \\ g+h \end{pmatrix} = \begin{pmatrix} 1 \\ 1 \\ 0 \end{pmatrix} \Rightarrow \quad \begin{cases} a+b = 1 & \text{(Eq. 1.1)} \\ d+e = 1 & \text{(Eq. 1.2)} \\ g+h = 0 & \text{(Eq. 1.3)} \end{cases}

Condition 2: A(101)=(101)A \begin{pmatrix} 1 \\ 0 \\ 1 \end{pmatrix} = \begin{pmatrix} -1 \\ 0 \\ 1 \end{pmatrix} Performing the matrix multiplication: (abcdefghi)(101)=(a(1)+b(0)+c(1)d(1)+e(0)+f(1)g(1)+h(0)+i(1))=(a+cd+fg+i)\begin{pmatrix} a & b & c \\ d & e & f \\ g & h & i \end{pmatrix} \begin{pmatrix} 1 \\ 0 \\ 1 \end{pmatrix} = \begin{pmatrix} a(1) + b(0) + c(1) \\ d(1) + e(0) + f(1) \\ g(1) + h(0) + i(1) \end{pmatrix} = \begin{pmatrix} a+c \\ d+f \\ g+i \end{pmatrix} Equating this to the given result:

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