3 and 4 .Determinants and Matrices
hard

For the system of linear equations $x+y+z=6$ ; $\alpha x+\beta y+7 z=3$ ; $x+2 y+3 z=14$ which of the following is $NOT$ true ?

A

If $\alpha=\beta=7$, then the system has no solution

B

If $\alpha=\beta$ and $\alpha \neq 7$ then the system has a unique solution.

C

There is a unique point $(\alpha, \beta)$ on the line $x +2 y +18=0$ for which the system has infinitely many solutions

D

For every point $(\alpha, \beta) \neq(7,7)$ on the line $x-2 y+7=0$, the system has infinitely many solutions.

(JEE MAIN-2023)

Solution

By equation $1$ and $3$

And $\begin{array}{c}y+2 z=8 \\ y=8-2 z \\ x=-2+z\end{array}$

Now putting in equation $2$

$\alpha(z-2)+\beta(-2 z+8)+7 z=3$

$\Rightarrow(\alpha-2 \beta+7) z=2 \alpha-8 \beta+3$

So equations have unique solution if $\alpha-2 \beta+7 \neq 0$

And equations have no solution if $\alpha-2 \beta+7=0$ and $2 \alpha-8 \beta+3 \neq 0$

And equations have infinite solution if $\alpha-2 \beta+7=0$ and $2 \alpha-8 \beta+3=0$

Standard 12
Mathematics

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