For the system of linear equations
$2x - y + 3z = 5$
$3x + 2y - z = 7$
$4x + 5y + \alpha z = \beta$,
which of the following is NOT correct?
Solution
Given system of linear equation is
<br/><br/>$$
\begin{gathered}
2 x-y+3 z=5 \\\\
3 x+2 y-z=7 \\\\
4 x+5 y+\alpha z=\beta \\\\
\text { Now, } \Delta=\left|\begin{array}{ccc}
2 & -1 & 3 \\
3 & 2 & -1 \\
4 & 5 & \alpha
\end{array}\right|=7(\alpha+5)
\end{gathered}
$$
<br/><br/>So, this system of equation has unique solution, if $\alpha \neq-5$
<br/><br/>and $\Delta_1=\left|\begin{array}{ccc}5 & -1 & 3 \\ 7 & 2 & -1 \\ \beta & 5 & \alpha\end{array}\right|=17 \alpha-5 \beta+30$
<br/><br/>and $\Delta_2=\left|\begin{array}{ccc}2 & 5 & 3 \\ 3 & 7 & -1 \\ 4 & \beta & \alpha\end{array}\right|=-11 \beta+\alpha+104$
<br/><br/>and $\Delta_3=\left|\begin{array}{ccc}2 & -1 & 5 \\ 3 & 2 & 7 \\ 4 & 5 & \beta\end{array}\right|=7(\beta-9)$
<br/><br/>For infinitely many solutions,
<br/><br/>$\Delta=\Delta_1=\Delta_2=\Delta_3=0$
<br/><br/>$$
\begin{aligned}
& \text { So, } \Delta=0 \\\\
& \Rightarrow 7(\alpha+5)=0 \\\\
& \Rightarrow \alpha=-5
\end{aligned}
$$
<br/><br/>$$
\begin{aligned}
& \Delta_3 =0 \\\\
& \Rightarrow 7(\beta-9) =0 \\\\
& \Rightarrow \beta =9
\end{aligned}
$$
<br/><br/>If $\alpha=-5$ and $\beta=8$, then $\Delta$ equals zero but $\Delta_3$ does not, which would imply the system is inconsistent for $\alpha=-5$ and $\beta=8$.
<br/><br/>Therefore, the option "The system is inconsistent for $\alpha=-5$ and $\beta=8$ " is correct.
About this question
Subject: Mathematics · Chapter: Matrices and Determinants · Topic: Types of Matrices and Operations
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