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Question

Let $\alpha, \beta \in \mathbf{R}$ be such that the system of linear equations
$x+2y+z=5$
$2x+y+\alpha z=5$
$8x+4y+\beta z=18$
has no solution. Then $\frac{\beta}{\alpha}$ is equal to :

The correct answer is
4

Linear Equations System Analysis

We are given a system of linear equations:

  • $x + 2y + z = 5$
  • $2x + y + \alpha z = 5$
  • $8x + 4y + \beta z = 18$

This system is stated to have no solution. We need to determine the value of the ratio $\frac{\beta}{\alpha}$.

System Equations Matrix Reduction

Represent the system using an augmented matrix and apply row operations to simplify it.

The initial augmented matrix is:

121|5
21$\alpha$|5
84$\beta$|18

Perform row operations $R_2 \rightarrow R_2 - 2R_1$ and $R_3 \rightarrow R_3 - 8R_1$:

121|5
0-3$\alpha - 2$|-5
0-12$\beta - 8$|-22

Perform row operation $R_3 \rightarrow R_3 - 4R_2$:

121|5
0-3$\alpha - 2$|-5
00$\beta - 4\alpha$|-2

No Solution Condition Derivation

For a system of linear equations represented by an augmented matrix to have no solution, the row-reduced form must contain a row equivalent to $[0 \ 0 \ \dots \ 0 \ | \ k]$, where $k \neq 0$.

Looking at the last row of our reduced matrix, it corresponds to the equation:

$(\beta - 4\alpha)z = -2$

This equation yields no solution if the coefficient of $z$ is zero and the constant term is non-zero.

  • Condition 1: $\beta - 4\alpha = 0$
  • Condition 2: $-2 \neq 0$ (This is always true)

Ratio Beta/Alpha Calculation

From the condition $\beta - 4\alpha = 0$, we can write:

$\beta = 4\alpha$

To find the ratio $\frac{\beta}{\alpha}$, we divide both sides by $\alpha$. This is permissible because the options provided are finite numbers, implying $\alpha \neq 0$.

$\frac{\beta}{\alpha} = \frac{4\alpha}{\alpha}$

$\implies \frac{\beta}{\alpha} = 4$

The value of the ratio $\frac{\beta}{\alpha}$ is 4.

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