- A0
- B1
- C2
- D3
View written solutionFree
Correct answer: B
We need to find for how many values of 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 can possibly make the system inconsistent.
3. Check consistency at
Substitute 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 .
4. Count the number of such values
There is exactly one value of 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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