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Question

Let $p = (x + y + z)$ and $q = xyz$. If $\begin{vmatrix} x & 1 & 1 \\ 1 & y & 1 \\ 1 & 1 & z \end{vmatrix}$ is positive, then which one of the following is correct ?

The correct answer is
$q + 2 > p$

Determinant Calculation

First, we evaluate the determinant of the given matrix:

Let $D = \begin{vmatrix} x & 1 & 1 \\ 1 & y & 1 \\ 1 & 1 & z \end{vmatrix}$. Expanding the determinant, we get: $D = x(yz - 1 \times 1) - 1(1 \times z - 1 \times 1) + 1(1 \times 1 - y \times 1)$ $D = x(yz - 1) - (z - 1) + (1 - y)$ $D = xyz - x - z + 1 + 1 - y$ $D = xyz - x - y - z + 2$

Applying Given Conditions

We are given that the determinant $D$ is positive:

$D > 0$

Substituting the calculated expression for $D$:

$xyz - x - y - z + 2 > 0$

Relating to p and q

We are also given the definitions $p = x + y + z$ and $q = xyz$. Substitute these into the inequality:

$q - (x + y + z) + 2 > 0$

$q - p + 2 > 0$

Final Inequality

Rearranging the terms to match the options:

$q + 2 > p$

This inequality matches Option C.

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Important Questions from Matrices and Determinants

  1. What is the value of the determinant of the inverse of the matrix $\begin{bmatrix} -4 & -5 \\ 2 & 2 \end{bmatrix}$?

  2. In obtaining the solution of the system of equations $x + y + z = 7$, $x + 2y+3z = 16$ and $x + 3y+4z = 22$ by Cramer's rule, the value of $y$ is obtained by dividing $D$ by $D_2$, where 

    $D = \begin{vmatrix} 1 & 1 & 1 \\ 1 & 2 & 3 \\ 1 & 3 & 4 \end{vmatrix}$ 

    What is the value of the determinant $D_2$?

  3. Consider the following in respect of non-singular matrices $A$ and $B$ : 

    I. $(AB)^{-1} = A^{-1}B^{-1}$ 

    II. $(BA)(AB)^{-1} = I$, where $I$ is the identity matrix 

    III. $(AB)^T = A^T B^T$ 

    How many of the above are correct?

  4. Consider the following statements : 

    Statement-I : 

    If $X$ is an $n \times n$ matrix, then $\det(mX) = m^n \det(X)$, where $m$ is a scalar. 

    Statement-II : 

    If $Y$ is a matrix obtained from $X$ by multiplying any row or column by a scalar $m$, then $\det(Y) = m\det(X)$. 

    Which one of the following is correct in respect of the above statements?

  5. Consider the following statements about the matrix $M = \begin{vmatrix} 71 & 23 & 48 \\ 57 & 28 & 29 \\ 65 & 17 & 48 \end{vmatrix}$ 

    Statement-I : The inverse of $M$ does not exist. 

    Statement-II : $M$ is non-singular. 

    Which one of the following is correct in respect of the above statements?

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