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Matrices and Determinants question

2022 · 28 Jun · Shift 1 · Q25
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Matrices and Determinants question

2022 · 28 Jun · Shift 1 · Q25

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
If the system of linear equations 2x+3y−z=−2x+y+z=4x−y+∣λ∣z=4λ−42x + 3y - z = - 2x + y + z = 4x - y + |\lambda |z = 4\lambda - 42x+3y−z=−2x+y+z=4x−y+∣λ∣z=4λ−4 where, λ∈\lambda\inλ∈ R, has no solution, then
  1. A
    λ\lambdaλ = 7
  2. B
    λ\lambdaλ=−-− 7
  3. C
    λ\lambdaλ = 8
  4. D
    λ\lambdaλ 2 = 1
View written solutionFree

Correct answer: B

We interpret the chained equality as the system

{2x+3y−z=4−2x+y+z=44x−y+∣λ∣z=4λ−4\begin{cases} 2x+3y-z = 4\\ -2x+y+z = 4\\ 4x-y+|\lambda|z = 4\lambda-4 \end{cases}⎩⎨⎧​2x+3y−z=4−2x+y+z=44x−y+∣λ∣z=4λ−4​

We need the value(s) of λ\lambdaλ for which the system has no solution.

1. Form the coefficient matrix

The coefficient matrix is

A=(23−1−2114−1∣λ∣)A=\begin{pmatrix} 2&3&-1\\ -2&1&1\\ 4&-1&|\lambda| \end{pmatrix}A=​2−24​31−1​−11∣λ∣​​

and the augmented matrix is

(23−14−21144−1∣λ∣4λ−4).\left(\begin{array}{ccc|c} 2&3&-1&4\\ -2&1&1&4\\ 4&-1&|\lambda|&4\lambda-4 \end{array}\right).​2−24​31−1​−11∣λ∣​444λ−4​​.

For a system to have no solution, we need:

  • det⁡(A)=0\det(A)=0det(A)=0 so that the system is singular, and
  • the augmented system must be inconsistent.

2. Compute det⁡(A)\det(A)det(A)

det⁡(A)=∣23−1−2114−1∣λ∣∣\det(A)= \begin{vmatrix} 2&3&-1\\ -2&1&1\\ 4&-1&|\lambda| \end{vmatrix}det(A)=​2−24​31−1​−11∣λ∣​​

Expand along the first row:

det⁡(A)=2∣11−1∣λ∣∣−3∣−214∣λ∣∣+(−1)∣−214−1∣\det(A)=2\begin{vmatrix}1&1\\-1&|\lambda|\end{vmatrix} -3\begin{vmatrix}-2&1\\4&|\lambda|\end{vmatrix} +(-1)\begin{vmatrix}-2&1\\4&-1\end{vmatrix}det(A)=2​1−1​1∣λ∣​​−3​−24​1∣λ∣​​+(−1)​−24​1−1​​ =2(∣λ∣+1)−3(−2∣λ∣−4)−((−2)(−1)−4)=2(|\lambda|+1)-3(-2|\lambda|-4)-((-2)(-1)-4)=2(∣λ∣+1)−3(−2∣λ∣−4)−((−2)(−1)−4) =2∣λ∣+2+6∣λ∣+12−(2−4)=2|\lambda|+2+6|\lambda|+12-(2-4)=2∣λ∣+2+6∣λ∣+12−(2−4) =8∣λ∣+14=8|\lambda|+14=8∣λ∣+14

This would never be zero, which suggests a sign issue in expansion. So let us compute carefully again.

Using direct expansion:

det⁡(A)=2(1⋅∣λ∣−1⋅(−1))−3((−2)⋅∣λ∣−1⋅4)+(−1)((−2)(−1)−1⋅4)\det(A)=2(1\cdot |\lambda|-1\cdot(-1)) -3((-2)\cdot |\lambda|-1\cdot 4) +(-1)((-2)(-1)-1\cdot 4)det(A)=2(1⋅∣λ∣−1⋅(−1))−3((−2)⋅∣λ∣−1⋅4)+(−1)((−2)(−1)−1⋅4) =2(∣λ∣+1)−3(−2∣λ∣−4)−1(2−4)=2(|\lambda|+1)-3(-2|\lambda|-4)-1(2-4)=2(∣λ∣+1)−3(−2∣λ∣−4)−1(2−4) =2∣λ∣+2+6∣λ∣+12+2=2|\lambda|+2+6|\lambda|+12+2=2∣λ∣+2+6∣λ∣+12+2 =8∣λ∣+16=8(∣λ∣+2)=8|\lambda|+16=8(|\lambda|+2)=8∣λ∣+16=8(∣λ∣+2)

Again impossible to vanish. This means the intended equations are more likely the standard interpretation from the printed question:

2x+3y−z=4,−2x+y+z=4,4x−y+∣λ∣z=4λ−4.2x+3y-z=4, \quad -2x+y+z=4, \quad 4x-y+|\lambda|z=4\lambda-4.2x+3y−z=4,−2x+y+z=4,4x−y+∣λ∣z=4λ−4.

Now reduce using the first two equations.

3. Solve first two equations

Add the first two equations:

(2x+3y−z)+(−2x+y+z)=4+4(2x+3y-z)+(-2x+y+z)=4+4(2x+3y−z)+(−2x+y+z)=4+4 4y=8⇒y=24y=8\Rightarrow y=24y=8⇒y=2

Substitute into the first equation:

2x+3(2)−z=42x+3(2)-z=42x+3(2)−z=4 2x−z=−22x-z=-22x−z=−2

So

z=2x+2.z=2x+2.z=2x+2.

4. Use the third equation

Third equation:

4x−y+∣λ∣z=4λ−44x-y+|\lambda|z=4\lambda-44x−y+∣λ∣z=4λ−4

With y=2y=2y=2 and z=2x+2z=2x+2z=2x+2,

4x−2+∣λ∣(2x+2)=4λ−44x-2+|\lambda|(2x+2)=4\lambda-44x−2+∣λ∣(2x+2)=4λ−4 4x−2+2∣λ∣x+2∣λ∣=4λ−44x-2+2|\lambda|x+2|\lambda|=4\lambda-44x−2+2∣λ∣x+2∣λ∣=4λ−4 (4+2∣λ∣)x=4λ−2−2∣λ∣(4+2|\lambda|)x = 4\lambda-2-2|\lambda|(4+2∣λ∣)x=4λ−2−2∣λ∣ (2+∣λ∣)x=2λ−1−∣λ∣.(2+|\lambda|)x = 2\lambda-1-|\lambda|.(2+∣λ∣)x=2λ−1−∣λ∣.

Since

2+∣λ∣>02+|\lambda|>02+∣λ∣>0

for every real λ\lambdaλ, there is always a unique value of xxx. Thus the system can never be inconsistent.

So there must be a typo in the question/options, or the intended third equation likely has coefficient λ\lambdaλ (not ∣λ∣|\lambda|∣λ∣) and/or different constants.

5. Check the option matching the usual intended form

A very common version is

4x−y+λz=4λ−4.4x-y+\lambda z=4\lambda-4.4x−y+λz=4λ−4.

Then using y=2, z=2x+2y=2,\ z=2x+2y=2, z=2x+2:

4x−2+λ(2x+2)=4λ−44x-2+\lambda(2x+2)=4\lambda-44x−2+λ(2x+2)=4λ−4 (4+2λ)x=2λ−2.(4+2\lambda)x = 2\lambda-2.(4+2λ)x=2λ−2.

For no solution, we need coefficient of xxx to be zero but RHS nonzero:

4+2λ=0⇒λ=−2,4+2\lambda=0 \Rightarrow \lambda=-2,4+2λ=0⇒λ=−2,

and then RHS =2(−2)−2=−6≠0=2(-2)-2=-6\neq 0=2(−2)−2=−6=0, so no solution at λ=−2\lambda=-2λ=−2. But −2-2−2 is not among the options.

Hence, as written, none of the options is correct.

6. Conclusion

The given system, with ∣λ∣|\lambda|∣λ∣ in the third equation, is always consistent and has a unique solution for every real λ\lambdaλ. Therefore it never has no solution.

So the stored answer B\text{B}B does not agree with the mathematics of the written question.

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