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Matrices and Determinants question

2025 · 2 Apr · Shift 1 · Q39
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  5. /2025 · 2 Apr · Shift 1 · Q39

Matrices and Determinants question

2025 · 2 Apr · Shift 1 · Q39

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
If the system of linear equations 3x+y+βz=32x+αy−z=−3x+2y+z=4\begin{aligned} & 3 x+y+\beta z=3 \\ & 2 x+\alpha y-z=-3 \\ & x+2 y+z=4 \end{aligned}​3x+y+βz=32x+αy−z=−3x+2y+z=4​ has infinitely many solutions, then the value of 22β−9α22 \beta-9 \alpha22β−9α is :
  1. A
    31
  2. B
    37
  3. C
    43
  4. D
    49
View written solutionFree

Correct answer: A

  1. For the system to have infinitely many solutions, we need:

    • The coefficient matrix to be singular: det⁡(A)=0\det(A)=0det(A)=0
    • And the system to be consistent, i.e. ranks of coefficient and augmented matrices must be equal and less than 3.

    The system is

    3x+y+\beta z&=3 \\ 2x+\alpha y-z&=-3 \\ x+2y+z&=4 \end{aligned}$$ So the coefficient matrix is $$A=\begin{pmatrix} 3&1&\beta\\ 2&\alpha&-1\\ 1&2&1 \end{pmatrix}$$ and the augmented matrix is $$\left(\begin{array}{ccc|c} 3&1&\beta&3\\ 2&\alpha&-1&-3\\ 1&2&1&4 \end{array}\right).$$
  2. Since infinitely many solutions require one equation to be a linear combination of the other two, let us use the third equation to express such dependence.

    Suppose R1=aR3+bR2R_1=aR_3+bR_2R1​=aR3​+bR2​ in the augmented sense.

    Comparing coefficients and constants: 3=a+2b3=a+2b3=a+2b 1=2a+αb1=2a+\alpha b1=2a+αb β=a−b\beta=a-bβ=a−b 3=4a−3b3=4a-3b3=4a−3b

  3. Solve for a,ba,ba,b using the first and fourth equations: a+2b=3a+2b=3a+2b=3 4a−3b=34a-3b=34a−3b=3

    From the first, a=3−2ba=3-2ba=3−2b

    Substitute into the second: 4(3−2b)−3b=34(3-2b)-3b=34(3−2b)−3b=3 12−8b−3b=312-8b-3b=312−8b−3b=3 12−11b=312-11b=312−11b=3 11b=911b=911b=9 b=911b=\frac{9}{11}b=119​

    Hence a=3−2⋅911=33−1811=1511.a=3-2\cdot\frac{9}{11}=\frac{33-18}{11}=\frac{15}{11}.a=3−2⋅119​=1133−18​=1115​.

  4. Now find α\alphaα and β\betaβ.

    From 1=2a+αb1=2a+\alpha b1=2a+αb we get 1=2⋅1511+α⋅9111=2\cdot\frac{15}{11}+\alpha\cdot\frac{9}{11}1=2⋅1115​+α⋅119​ 1=3011+9α111=\frac{30}{11}+\frac{9\alpha}{11}1=1130​+119α​ 11=30+9α11=30+9\alpha11=30+9α 9α=−199\alpha=-199α=−19 α=−199.\alpha=-\frac{19}{9}.α=−919​.

    From β=a−b\beta=a-bβ=a−b β=1511−911=611.\beta=\frac{15}{11}-\frac{9}{11}=\frac{6}{11}.β=1115​−119​=116​.

  5. Compute the required value: 22β−9α=22⋅611−9(−199)22\beta-9\alpha=22\cdot\frac{6}{11}-9\left(-\frac{19}{9}\right)22β−9α=22⋅116​−9(−919​) =12+19=31.=12+19=31.=12+19=31.

  6. Therefore, the correct option is 31\boxed{31}31​ which is Option A.

  7. Comparison with stored answer:

    Stored correct answer = A

    Our derived answer = A

    So they agree.

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