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Matrices and Determinants question

2022 · 24 Jun · Shift 1 · Q26
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  5. /2022 · 24 Jun · Shift 1 · Q26

Matrices and Determinants question

2022 · 24 Jun · Shift 1 · Q26

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
The number of values of α\alphaα for which the system of equations : x + y + z =αα\alpha\alphaαα x + 2 α\alphaα y + 3z = −-− 1 x + 3 α\alphaα y + 5z = 4 is inconsistent, is
  1. A
    0
  2. B
    1
  3. C
    2
  4. D
    3
View written solutionFree

Correct answer: B

We need to find for how many values of α\alphaα the given system is inconsistent.

1. Interpret the system correctly

The equations are: [ \begin{cases} x+y+z=\alpha \ \alpha x+2\alpha y+3z=-1 \ x+3\alpha y+5z=4 \end{cases} ]

So the coefficient matrix is [ A=\begin{pmatrix} 1&1&1\ \alpha&2\alpha&3\ 1&3\alpha&5 \end{pmatrix} ] and the augmented column is [ B=\begin{pmatrix} \alpha\-1\4 \end{pmatrix}. ]

A system can be inconsistent only when the determinant of the coefficient matrix is zero.


2. Compute the determinant

[ \Delta=\begin{vmatrix} 1&1&1\ \alpha&2\alpha&3\ 1&3\alpha&5 \end{vmatrix} ]

Expand along the first row: [ \Delta=1\begin{vmatrix}2\alpha&3\3\alpha&5\end{vmatrix} -1\begin{vmatrix}\alpha&3\1&5\end{vmatrix} +1\begin{vmatrix}\alpha&2\alpha\1&3\alpha\end{vmatrix} ]

Now evaluate each minor: [ \begin{vmatrix}2\alpha&3\3\alpha&5\end{vmatrix}=10\alpha-9\alpha=\alpha ] [ \begin{vmatrix}\alpha&3\1&5\end{vmatrix}=5\alpha-3 ] [ \begin{vmatrix}\alpha&2\alpha\1&3\alpha\end{vmatrix}=3\alpha^2-2\alpha ]

Hence, [ \Delta=\alpha-(5\alpha-3)+(3\alpha^2-2\alpha) ] [ \Delta=\alpha-5\alpha+3+3\alpha^2-2\alpha ] [ \Delta=3\alpha^2-6\alpha+3 ] [ \Delta=3(\alpha^2-2\alpha+1)=3(\alpha-1)^2 ]

So, [ \Delta=0 \iff \alpha=1 ]

Thus only α=1\alpha=1α=1 can possibly make the system inconsistent.


3. Check consistency at α=1\alpha=1α=1

Substitute α=1\alpha=1α=1 into the system: [ \begin{cases} x+y+z=1 \ x+2y+3z=-1 \ x+3y+5z=4 \end{cases} ]

Now compare equations.

From second minus first: [ (x+2y+3z)-(x+y+z)=-1-1 ] [ y+2z=-2 \quad ...(1) ]

From third minus second: [ (x+3y+5z)-(x+2y+3z)=4-(-1) ] [ y+2z=5 \quad ...(2) ]

Equations (1) and (2) give [ y+2z=-2 \quad \text{and} \quad y+2z=5 ] which is impossible.

Hence the system is inconsistent for α=1\alpha=1α=1.


4. Count the number of such values

There is exactly one value of α\alphaα for which the system is inconsistent.

So the correct option is: [ \boxed{\text{B: }1} ]


5. Compare with stored answer

Stored correct answer: B

Our derived answer: B

They agree.

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