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Verify that the product of two symmetric matrices, [tex]\(AB\)[/tex], is also symmetric. To do this, calculate [tex]\(AB\)[/tex] and then [tex]\(B^{\top}A^{\top}\)[/tex].

a) Given:
[tex]\[ A=\begin{bmatrix} 2 & 0 \\ 0 & 5 \end{bmatrix}, \quad B=\begin{bmatrix} 3 & 0 \\ 0 & -10 \end{bmatrix} \][/tex]
Calculate the multiplication of [tex]\(A\)[/tex] and [tex]\(B\)[/tex], then take the transpose of the result and verify symmetry.

b) Given:
[tex]\[ A=\begin{bmatrix} 0 & -5 \\ -5 & 1 \end{bmatrix}, \quad B=\begin{bmatrix} 1 & 2 \\ 2 & 0 \end{bmatrix} \][/tex]
Calculate the multiplication of [tex]\(A\)[/tex] and [tex]\(B\)[/tex], then take the transpose of the result and verify symmetry.


Sagot :

Claro, vamos a realizar la verificación de que [tex]\( AB \)[/tex] es simétrica para las dos parejas de matrices [tex]\( A \)[/tex] y [tex]\( B \)[/tex].

### Parte (a)
Dadas las matrices:
[tex]\[ A = \begin{array}{cc} 2 & 0 \\ 0 & 5 \end{array} \][/tex]
[tex]\[ B = \begin{array}{cc} 3 & 0 \\ 0 & -10 \end{array} \][/tex]

Paso 1: Calculemos [tex]\( AB \)[/tex].

[tex]\[ AB = \begin{array}{cc} 2 & 0 \\ 0 & 5 \end{array} \begin{array}{cc} 3 & 0 \\ 0 & -10 \end{array} = \begin{array}{cc} (2 \cdot 3 + 0 \cdot 0) & (2 \cdot 0 + 0 \cdot -10) \\ (0 \cdot 3 + 5 \cdot 0) & (0 \cdot 0 + 5 \cdot -10) \end{array} = \begin{array}{cc} 6 & 0 \\ 0 & -50 \end{array} \][/tex]

Paso 2: Calculemos [tex]\( B^{\top} A^{\top} \)[/tex].

Primero tomamos las transpuestas:
[tex]\[ A^{\top} = \begin{array}{cc} 2 & 0 \\ 0 & 5 \end{array} \][/tex] (como A es simétrica, A = [tex]\( A^{\top} \)[/tex])
[tex]\[ B^{\top} = \begin{array}{cc} 3 & 0 \\ 0 & -10 \end{array} \][/tex] (como B es simétrica, B = [tex]\( B^{\top} \)[/tex])

Luego calculamos el producto:
[tex]\[ B^{\top} A^{\top} = \begin{array}{cc} 3 & 0 \\ 0 & -10 \end{array} \begin{array}{cc} 2 & 0 \\ 0 & 5 \end{array} = \begin{array}{cc} (3 \cdot 2 + 0 \cdot 0) & (3 \cdot 0 + 0 \cdot 5) \\ (0 \cdot 2 + -10 \cdot 0) & (0 \cdot 0 + -10 \cdot 5) \end{array} = \begin{array}{cc} 6 & 0 \\ 0 & -50 \end{array} \][/tex]

Paso 3: Comparación

Comparando los resultados de [tex]\( AB \)[/tex] y [tex]\( B^{\top} A^{\top} \)[/tex]:

[tex]\[ AB = \begin{array}{cc} 6 & 0 \\ 0 & -50 \end{array} \][/tex]
[tex]\[ B^{\top} A^{\top} = \begin{array}{cc} 6 & 0 \\ 0 & -50 \end{array} \][/tex]

Los dos son iguales, por lo que [tex]\( AB \)[/tex] es simétrica.

### Parte (b)
Dadas las matrices:
[tex]\[ A = \begin{array}{cc} 0 & -5 \\ -5 & 1 \end{array} \][/tex]
[tex]\[ B = \begin{array}{cc} 1 & 2 \\ 2 & 0 \end{array} \][/tex]

Paso 1: Calculemos [tex]\( AB \)[/tex].

[tex]\[ AB = \begin{array}{cc} 0 & -5 \\ -5 & 1 \end{array} \begin{array}{cc} 1 & 2 \\ 2 & 0 \end{array} = \begin{array}{cc} (0 \cdot 1 + -5 \cdot 2) & (0 \cdot 2 + -5 \cdot 0) \\ (-5 \cdot 1 + 1 \cdot 2) & (-5 \cdot 2 + 1 \cdot 0) \end{array} = \begin{array}{cc} -10 & 0 \\ -3 & -10 \end{array} \][/tex]

Paso 2: Calculemos [tex]\( B^{\top} A^{\top} \)[/tex].

Primero las transpuestas:
[tex]\[ A^{\top} = \begin{array}{cc} 0 & -5 \\ -5 & 1 \end{array} \][/tex] (como A es simétrica, A = [tex]\( A^{\top} \)[/tex])
[tex]\[ B^{\top} = \begin{array}{cc} 1 & 2 \\ 2 & 0 \end{array} \][/tex] (como B es simétrica, B = [tex]\( B^{\top} \)[/tex])

Luego calculamos el producto:
[tex]\[ B^{\top} A^{\top} = \begin{array}{cc} 1 & 2 \\ 2 & 0 \end{array} \begin{array}{cc} 0 & -5 \\ -5 & 1 \end{array} = \begin{array}{cc} (1 \cdot 0 + 2 \cdot -5) & (1 \cdot -5 + 2 \cdot 1) \\ (2 \cdot 0 + 0 \cdot -5) & (2 \cdot -5 + 0 \cdot 1) \end{array} = \begin{array}{cc} -10 & -3 \\ 0 & -10 \end{array} \][/tex]

Paso 3: Comparación

Comparando los resultados de [tex]\( AB \)[/tex] y [tex]\( B^{\top} A^{\top} \)[/tex]:

[tex]\[ AB = \begin{array}{cc} -10 & 0 \\ -3 & -10 \end{array} \][/tex]
[tex]\[ B^{\top} A^{\top} = \begin{array}{cc} -10 & -3 \\ 0 & -10 \end{array} \][/tex]

Los dos no son iguales, por lo que [tex]\( AB \)[/tex] no es simétrica.

### Resumen
- En la parte (a), [tex]\( AB \)[/tex] resultó simétrica.
- En la parte (b), [tex]\( AB \)[/tex] no resultó simétrica.