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If xyz=m and det p=|{:(x,y,z),(z,x,y),(y...

If xyz=m and det `p=|{:(x,y,z),(z,x,y),(y,z,x):}|` , where p is an orthogonal matrix.
The value of `x^3+y^3+z^3` can be

A

6m `+- 1`

B

` 3m +-1`

C

`+-3m `

D

none of these

Text Solution

AI Generated Solution

The correct Answer is:
To solve the problem step by step, we will analyze the given information and derive the required expression for \( x^3 + y^3 + z^3 \). ### Step 1: Understand the Matrix and Its Properties We are given the matrix \( P \): \[ P = \begin{pmatrix} x & y & z \\ z & x & y \\ y & z & x \end{pmatrix} \] Since \( P \) is an orthogonal matrix, we know that \( P^T P = I \) (the identity matrix). ### Step 2: Compute \( P^T \) The transpose of matrix \( P \) is: \[ P^T = \begin{pmatrix} x & z & y \\ y & x & z \\ z & y & x \end{pmatrix} \] ### Step 3: Compute \( P P^T \) We need to calculate \( P P^T \): \[ P P^T = \begin{pmatrix} x & y & z \\ z & x & y \\ y & z & x \end{pmatrix} \begin{pmatrix} x & z & y \\ y & x & z \\ z & y & x \end{pmatrix} \] Calculating the elements of the resulting matrix: - The (1,1) entry: \( x^2 + y^2 + z^2 \) - The (1,2) entry: \( xy + xz + yz \) - The (1,3) entry: \( xz + yx + zy \) - The (2,1) entry: \( zx + xy + yz \) - The (2,2) entry: \( z^2 + x^2 + y^2 \) - The (2,3) entry: \( zy + zx + xy \) - The (3,1) entry: \( yx + zy + xz \) - The (3,2) entry: \( y^2 + z^2 + x^2 \) - The (3,3) entry: \( z^2 + y^2 + x^2 \) This gives us: \[ P P^T = \begin{pmatrix} x^2 + y^2 + z^2 & xy + xz + yz & xy + xz + yz \\ xy + xz + yz & x^2 + y^2 + z^2 & xy + xz + yz \\ xy + xz + yz & xy + xz + yz & x^2 + y^2 + z^2 \end{pmatrix} \] ### Step 4: Set Equal to Identity Matrix Since \( P P^T = I \), we have: 1. \( x^2 + y^2 + z^2 = 1 \) 2. \( xy + xz + yz = 0 \) ### Step 5: Use the Identity for Cubes We know the identity: \[ x^3 + y^3 + z^3 - 3xyz = (x + y + z)(x^2 + y^2 + z^2 - xy - xz - yz) \] Substituting the known values: - \( x^2 + y^2 + z^2 = 1 \) - \( xy + xz + yz = 0 \) This simplifies to: \[ x^3 + y^3 + z^3 - 3xyz = (x + y + z)(1 - 0) = x + y + z \] Thus: \[ x^3 + y^3 + z^3 = x + y + z + 3xyz \] ### Step 6: Find \( x + y + z \) From the earlier steps, since \( x^2 + y^2 + z^2 = 1 \) and \( xy + xz + yz = 0 \), we can conclude: \[ (x + y + z)^2 = x^2 + y^2 + z^2 + 2(xy + xz + yz) = 1 + 2(0) = 1 \] Thus, \( x + y + z = \pm 1 \). ### Step 7: Substitute Back Now substituting back: - If \( x + y + z = 1 \): \[ x^3 + y^3 + z^3 = 1 + 3xyz \] - If \( x + y + z = -1 \): \[ x^3 + y^3 + z^3 = -1 + 3xyz \] ### Conclusion Since \( xyz = m \): \[ x^3 + y^3 + z^3 = 3m \pm 1 \] Thus, the final answer is: \[ \boxed{3m \pm 1} \]
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