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Matrices and Determinants question

2023 · 11 Apr · Shift 2 · Q30
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Matrices and Determinants question

2023 · 11 Apr · Shift 2 · Q30

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
∣x+1xxxx+λxxxx+λ2∣=98(103x+81)\left|\begin{array}{ccc}x+1 & x & x \\ x & x+\lambda & x \\ x & x & x+\lambda^{2}\end{array}\right|=\frac{9}{8}(103 x+81)​x+1xx​xx+λx​xxx+λ2​​=89​(103x+81), then λ,λ3\lambda, \frac{\lambda}{3}λ,3λ​ are the roots of the equation :
  1. A
    4x2+24x−27=04 x^{2}+24 x-27=04x2+24x−27=0
  2. B
    4x2−24x+27=04 x^{2}-24 x+27=04x2−24x+27=0
  3. C
    4x2−24x−27=04 x^{2}-24 x-27=04x2−24x−27=0
  4. D
    4x2+24x+27=04 x^{2}+24 x+27=04x2+24x+27=0
View written solutionFree

Correct answer: B

  1. Interpret the determinant equation

We have

∣x+1xxxx+λxxxx+λ2∣=98(103x+81).\begin{vmatrix} x+1 & x & x\\ x & x+\lambda & x\\ x & x & x+\lambda^2 \end{vmatrix} = \frac{9}{8}(103x+81).​x+1xx​xx+λx​xxx+λ2​​=89​(103x+81).

We first simplify the determinant.


  1. Write the matrix in a convenient form

Observe that the matrix is

(x+1xxxx+λxxxx+λ2)=(1000λ000λ2)+(xxxxxxxxx).\begin{pmatrix} x+1 & x & x\\ x & x+\lambda & x\\ x & x & x+\lambda^2 \end{pmatrix} = \begin{pmatrix} 1 & 0 & 0\\ 0 & \lambda & 0\\ 0 & 0 & \lambda^2 \end{pmatrix} + \begin{pmatrix} x & x & x\\ x & x & x\\ x & x & x \end{pmatrix}.​x+1xx​xx+λx​xxx+λ2​​=​100​0λ0​00λ2​​+​xxx​xxx​xxx​​.

So this is of the form D+xJ,D + xJ,D+xJ, where D=diag⁡(1,λ,λ2),J=(111111111).D=\operatorname{diag}(1,\lambda,\lambda^2), \qquad J=\begin{pmatrix}1&1&1\\1&1&1\\1&1&1\end{pmatrix}.D=diag(1,λ,λ2),J=​111​111​111​​.

But it is easier here to expand directly.


  1. Compute the determinant

Let

Δ=∣x+1xxxx+λxxxx+λ2∣.\Delta= \begin{vmatrix} x+1 & x & x\\ x & x+\lambda & x\\ x & x & x+\lambda^2 \end{vmatrix}.Δ=​x+1xx​xx+λx​xxx+λ2​​.

Apply row operations: R2→R2−R1,R3→R3−R1.R_2 \to R_2-R_1, \qquad R_3 \to R_3-R_1.R2​→R2​−R1​,R3​→R3​−R1​. Then

Δ=∣x+1xx−1λ0−10λ2∣.\Delta= \begin{vmatrix} x+1 & x & x\\ -1 & \lambda & 0\\ -1 & 0 & \lambda^2 \end{vmatrix}.Δ=​x+1−1−1​xλ0​x0λ2​​.

Now expand along the first row:

Δ=(x+1)∣λ00λ2∣−x∣−10−1λ2∣+x∣−1λ−10∣.\Delta=(x+1) \begin{vmatrix} \lambda & 0\\ 0 & \lambda^2 \end{vmatrix} -x \begin{vmatrix} -1 & 0\\ -1 & \lambda^2 \end{vmatrix} +x \begin{vmatrix} -1 & \lambda\\ -1 & 0 \end{vmatrix}.Δ=(x+1)​λ0​0λ2​​−x​−1−1​0λ2​​+x​−1−1​λ0​​.

Compute each minor:

∣λ00λ2∣=λ3,\begin{vmatrix} \lambda & 0\\ 0 & \lambda^2 \end{vmatrix}=\lambda^3,​λ0​0λ2​​=λ3, ∣−10−1λ2∣=−λ2,\begin{vmatrix} -1 & 0\\ -1 & \lambda^2 \end{vmatrix}=-\lambda^2,​−1−1​0λ2​​=−λ2, ∣−1λ−10∣=λ.\begin{vmatrix} -1 & \lambda\\ -1 & 0 \end{vmatrix}=\lambda.​−1−1​λ0​​=λ.

Hence

Δ=(x+1)λ3−x(−λ2)+xλ=λ3x+λ3+xλ2+xλ.\Delta=(x+1)\lambda^3-x(-\lambda^2)+x\lambda =\lambda^3 x+\lambda^3+x\lambda^2+x\lambda.Δ=(x+1)λ3−x(−λ2)+xλ=λ3x+λ3+xλ2+xλ.

So

Δ=x(λ3+λ2+λ)+λ3.\Delta=x(\lambda^3+\lambda^2+\lambda)+\lambda^3.Δ=x(λ3+λ2+λ)+λ3.
  1. Compare coefficients with the given RHS

Given

Δ=98(103x+81)=9278x+7298.\Delta=\frac{9}{8}(103x+81)=\frac{927}{8}x+\frac{729}{8}.Δ=89​(103x+81)=8927​x+8729​.

Therefore,

λ3=7298.\lambda^3=\frac{729}{8}.λ3=8729​.

So

λ=92.\lambda=\frac{9}{2}.λ=29​.

Also,

λ3+λ2+λ=9278.\lambda^3+\lambda^2+\lambda=\frac{927}{8}.λ3+λ2+λ=8927​.

Check with λ=92\lambda=\frac92λ=29​:

λ3+λ2+λ=7298+814+92=729+162+368=9278,\lambda^3+\lambda^2+\lambda =\frac{729}{8}+\frac{81}{4}+\frac92 =\frac{729+162+36}{8} =\frac{927}{8},λ3+λ2+λ=8729​+481​+29​=8729+162+36​=8927​,

which matches.

So the two numbers are

λ=92,λ3=32.\lambda=\frac92, \qquad \frac{\lambda}{3}=\frac32.λ=29​,3λ​=23​.
  1. Form the quadratic whose roots are λ\lambdaλ and λ/3\lambda/3λ/3

Roots are

92,32.\frac92, \frac32.29​,23​.

Their sum is

92+32=6,\frac92+\frac32=6,29​+23​=6,

and product is

92⋅32=274.\frac92\cdot\frac32=\frac{27}{4}.29​⋅23​=427​.

Hence the quadratic is

x2−6x+274=0.x^2-6x+\frac{27}{4}=0.x2−6x+427​=0.

Multiplying by 4,

4x2−24x+27=0.4x^2-24x+27=0.4x2−24x+27=0.
  1. Match with the options

This corresponds to:

Option B: 4x2−24x+27=0.4x^2-24x+27=0.4x2−24x+27=0.


  1. Compare with stored correct answer

Stored correct answer is B, which matches our derived answer.

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