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If A=[2−34−35x430]A = \begin{bmatrix} 2 & -3 & 4 \\ -3 & 5 & x \\ 4 & 3 & 0 \end{bmatrix} is a symmetric matrix and B=[02−10−2z6y−60]B = \begin{bmatrix} 0 & 2 & -10 \\ -2 & z & 6 \\ y & -6 & 0 \end{bmatrix} is a skew-symmetric matrix, then the value of (xy+yz+zx)(xy + yz + zx) is

Solution

✅ Correct Option: 2

Matrix A=[2−34−35x430]A = \begin{bmatrix} 2 & -3 & 4 \\ -3 & 5 & x \\ 4 & 3 & 0 \end{bmatrix} is symmetric.

For a symmetric matrix, aij=ajia_{ij} = a_{ji} for all elements.

Comparing position (2,3) with position (3,2):

Element at (2,3) = xx

Element at (3,2) = 33

Therefore x=3x = 3


Matrix B=[02−10−2z6y−60]B = \begin{bmatrix} 0 & 2 & -10 \\ -2 & z & 6 \\ y & -6 & 0 \end{bmatrix} is skew-symmetric.

For a skew-symmetric matrix, all diagonal elements equal 0.

The diagonal elements are: 0,z,00, z, 0

Therefore z=0z = 0


For a skew-symmetric matrix, bij=−bjib_{ij} = -b_{ji} for all elements.

Comparing position (1,3) with position (3,1):

Element at (1,3) = −10-10

Element at (3,1) = yy

−10=−y-10 = -y

Therefore y=10y = 10


The values are:

x=3x = 3

y=10y = 10

z=0z = 0

Now calculating:

xy+yz+zxxy + yz + zx

=(3)(10)+(10)(0)+(0)(3)= (3)(10) + (10)(0) + (0)(3)

=30+0+0= 30 + 0 + 0

=30= 30

Therefore, the value of (xy+yz+zx)=30(xy + yz + zx) = 30

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