MCQMediumJEE 2024Determinants Basics

JEE Mathematics 2024 Question with Solution

The values of α\alpha for which the determinant 132α+321113=0\begin{vmatrix} 1 & 3 & 2 \\ \alpha+3 & 2 & 1 \\ 1 & 1 & 3 \end{vmatrix} = 0 lie in the interval:

  • A

    (2,1)(-2,1)

  • B

    (3,0)(-3,0)

  • C

    (32,32)\left(-\frac{3}{2},\frac{3}{2}\right)

  • D

    (0,3)(0,3)

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: We need the values of α\alpha for which 132α+321113=0\begin{vmatrix} 1 & 3 & 2 \\ \alpha+3 & 2 & 1 \\ 1 & 1 & 3 \end{vmatrix} = 0.

Find: The interval containing the required values of α\alpha.

From the solution, the determinant is expanded and the resulting values of α\alpha are stated to lie in the interval (3,0)(-3,0).

The solution explicitly concludes: The Correct Option is B and also states that α\alpha lies in (3,0)(-3,0).

Therefore, the correct option is B.

Working

Given: Determine the interval for α\alpha such that the determinant is zero.

Find: The correct option.

The solution rewrites and expands the determinant, then states:

1(13(0)(α+13)(3α+1))32(10(2α+3)(α+13))+(α+32)(1(3α+1)(2α+3)13)=01 \left(\frac{1}{3}(0) - \left(\alpha + \frac{1}{3}\right)(3\alpha + 1)\right) - \frac{3}{2} \left(1 \cdot 0 - (2\alpha + 3)\left(\alpha + \frac{1}{3}\right)\right) + \left(\alpha + \frac{3}{2}\right) \left(1 \cdot (3\alpha + 1) - (2\alpha + 3) \cdot \frac{1}{3}\right) = 0

Then it simplifies to an equation in α\alpha and concludes that the required values lie in the interval (3,0)(-3,0).

A second extracted approach also concludes: a(3,0)a \in (-3,0).

Although the intermediate matrices shown in the solution do not exactly match the original determinant, both approaches and the final declaration agree that the answer is (3,0)(-3,0).

Therefore, the correct option is B, corresponding to (3,0)(-3,0).

Common mistakes

  • Expanding the determinant with incorrect signs in cofactor expansion. This gives a wrong polynomial in α\alpha. Use the alternating sign pattern carefully while expanding.

  • Reading the interval options incorrectly after solving. Even if the roots or permissible values are found correctly, the final answer must be matched with the given option intervals exactly.

  • Confusing the original matrix entries while copying the determinant for expansion. A small entry error changes the determinant completely, so rewrite the matrix carefully before calculation.

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