3 and 4 .Determinants and Matrices
hard

For the system of linear equations

$2 x-y+3 z=5$

$3 x+2 y-z=7$

$4 x+5 y+\alpha z=\beta$

Which of the following is NOT correct ?

A

The system has infinitely many solutions for $\alpha=-5$ and $\beta=9$

B

The system has a unique solution for $\alpha \neq-5$ and $\beta=8$

C

The system has infinitely many solutions for $\alpha=-6$ and $\beta=9$

D

The system is inconsistent for $\alpha=-5$ and $\beta=8$

(JEE MAIN-2023)

Solution

$\Delta=\left|\begin{array}{ccc}2 & -1 & 3 \\ 3 & 2 & -1 \\ 4 & 5 & \alpha\end{array}\right|=7(\alpha+5)$

$\Delta_1=\left|\begin{array}{ccc}5 & -1 & 3 \\ 7 & 2 & -1 \\ \beta & 5 & \alpha\end{array}\right|=17 \alpha-5 \beta+130$

$\Delta_2=\left|\begin{array}{ccc}2 & 5 & 3 \\ 3 & 7 & -1 \\ 4 & \beta & \alpha\end{array}\right|=-11 \beta+\alpha+104$

$\Delta_3=\left|\begin{array}{ccc}2 & -1 & 5 \\ 3 & 2 & 7 \\ 4 & 5 & \beta\end{array}\right|=7(\beta-9)$

For infinitely many solutions

$\Delta=\Delta_1=\Delta_2=\Delta_3=0$

For $\alpha=-5$ and $\beta=9$

Hence option $(3)$ is incorrect

Standard 12
Mathematics

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