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Question

The system of linear equations

x + 2y + z = 4, 2x + 4y + 2z = 8 and 3x + 6y + 3z = 10 has  

The correct answer is

no solution

\[ \text{Consider the system} \begin{aligned} x + 2y + z &= 4,\\ 2x + 4y + 2z &= 8,\\ 3x + 6y + 3z &= 10. \end{aligned} \] \[ \text{Observe that the second equation is }2\text{ times the first: }(2x+4y+2z)=2(x+2y+z)=8. \] \[ \text{If the third were consistent with the first, it would be }3(x+2y+z)=3\cdot 4=12, \] \[ \text{but the third equation has right-hand side }10\neq 12, \] \[ \text{so the three equations are inconsistent.} \] \[ \text{Augmented matrix row-reduction (illustration):} \] \[ \left[\begin{array}{ccc|c} 1 & 2 & 1 & 4\\[4pt] 2 & 4 & 2 & 8\\[4pt] 3 & 6 & 3 & 10 \end{array}\right] \overset{R_2\leftarrow R_2-2R_1}{\longrightarrow} \overset{R_3\leftarrow R_3-3R_1}{\longrightarrow} \left[\begin{array}{ccc|c} 1 & 2 & 1 & 4\\[4pt] 0 & 0 & 0 & 0\\[4pt] 0 & 0 & 0 & -2 \end{array}\right]. \] The last row $[0\;0\;0\;|\;-2]$ is a contradiction, so the system has no solution. \[ \boxed{\text{Option 2: no solution}} \] 

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Important Questions from Matrices

  1. Let A be a skew-symmetric matrix of order 3.

    What is the value of det(4A4) - det(3A3) + det(2A2) - det(A) + det(-I)  where I is the identity matrix of order 3?

  2. Let AX = B be a system of 3 linear equations with 3-unknowns. Let X 1 and X 2  be its two distinct solutions. If the combination  aX 1  + bX 2  is a solution of AX = B; where a, b are real numbers, then which one of the following is correct ? 
  3. An ordered pair $(\alpha, \beta)$ for which the system of linear equations

    $\alpha x + (\beta+1)y + z = 2$
    $2\alpha x + (\beta+2)y + z = 3$
    $\alpha x + \beta y + 2z = 2$ has a unique solution, is

  4. Let A and B be two non zero square matrics and AB and BA both are defined. It means

  5. For what value of k is the matrix \(\begin{bmatrix} 2\cos 2\theta & 2\cos 2\theta & 6 \\ 1 -2 \sin^2\theta & 2 \cos^2\theta -1 & 3 \\ k & 2k & 1 \end{bmatrix}\)  singular?

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