\( [10 \mathrm{p}] \) Calcule la matriz \( X \) que satisface: \[ (A X)^{-1}=A B \text {, siendo } A=\left(\begin{array}{cc}-1 & 1 \\ 1 & 0\end{array}\right) \quad y \quad B=\left(\begin{array}{cc}-1 & 1 \\ 1 & -2\end{array}\right) \]
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Para encontrar la matriz \( X \) que satisface la ecuación \( (A X)^{-1}=A B \), primero despejamos \( A X \): \[ A X = (A B)^{-1}. \] Ahora, necesitamos calcular \( A B \): \[ A B = \left(\begin{array}{cc}-1 & 1 \\ 1 & 0\end{array}\right) \left(\begin{array}{cc}-1 & 1 \\ 1 & -2\end{array}\right) = \left(\begin{array}{cc}1-1 & -1+(-2) \\ -1 & 1\end{array}\right) = \left(\begin{array}{cc}0 & -3 \\ -1 & 1\end{array}\right). \] Ahora que tenemos \( A B \), calculamos \( (A B)^{-1} \). Para encontrar la inversa de una matriz \( M = \left(\begin{array}{cc}a & b \\ c & d\end{array}\right) \) se usa la fórmula: \[ M^{-1} = \frac{1}{ad - bc} \left(\begin{array}{cc}d & -b \\ -c & a\end{array}\right). \] En nuestro caso: \[ a = 0, \quad b = -3, \quad c = -1, \quad d = 1. \] El determinante \( ad - bc = (0)(1) - (-3)(-1) = -3 \). Calculemos la inversa: \[ (A B)^{-1} = \frac{1}{-3} \left(\begin{array}{cc}1 & 3 \\ 1 & 0\end{array}\right) = \left(\begin{array}{cc}-\frac{1}{3} & -1 \\ -\frac{1}{3} & 0\end{array}\right). \] Ahora tenemos que: \[ A X = \left(\begin{array}{cc}-\frac{1}{3} & -1 \\ -\frac{1}{3} & 0\end{array}\right). \] Ahora, para encontrar \( X \), multiplicamos ambos lados por \( A^{-1} \): Primero, encontramos \( A^{-1} \). Para \( A = \left(\begin{array}{cc}-1 & 1 \\ 1 & 0\end{array}\right) \): El determinante de \( A \) es: \[ (-1)(0) - (1)(1) = -1. \] Entonces, la inversa es: \[ A^{-1} = \frac{1}{-1} \left(\begin{array}{cc}0 & -1 \\ 1 & -1\end{array}\right) = \left(\begin{array}{cc}0 & 1 \\ -1 & 1\end{array}\right). \] Multiplicamos \( A^{-1} \) por \( A X \): \[ X = A^{-1} \left(\begin{array}{cc}-\frac{1}{3} & -1 \\ -\frac{1}{3} & 0\end{array}\right) = \left(\begin{array}{cc}0 & 1 \\ -1 & 1\end{array}\right) \left(\begin{array}{cc}-\frac{1}{3} & -1 \\ -\frac{1}{3} & 0\end{array}\right). \] Realizamos la multiplicación: \[ X = \left(\begin{array}{cc}0 \cdot -\frac{1}{3} + 1 \cdot -\frac{1}{3} & 0 \cdot -1 + 1 \cdot 0 \\ -1 \cdot -\frac{1}{3} + 1 \cdot -\frac{1}{3} & -1 \cdot -1 + 1 \cdot 0\end{array}\right) = \left(\begin{array}{cc}-\frac{1}{3} & 0 \\ 0 & 1\end{array}\right). \] Por lo tanto, la matriz \( X \) es: \[ X = \left(\begin{array}{cc}-\frac{1}{3} & 0 \\ 0 & 1\end{array}\right). \]