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Exercise 4.6 · Q130

Q.A=[243−132]A=\begin{bmatrix}2&4&3\\-1&3&2\end{bmatrix}, B=[2−233−11]B=\begin{bmatrix}2&-2\\3&3\\-1&1\end{bmatrix} and C=[3113]C=\begin{bmatrix}3&1\\1&3\end{bmatrix}.

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Given A=[243−132]A=\begin{bmatrix}2&4&3\\-1&3&2\end{bmatrix}, B=[2−233−11]B=\begin{bmatrix}2&-2\\3&3\\-1&1\end{bmatrix}, C=[3113]C=\begin{bmatrix}3&1\\1&3\end{bmatrix}.

Step 1 — compute AB (2×32\times3 times 3×23\times2):

Row 1: [2(2)+4(3)+3(−1), 2(−2)+4(3)+3(1)]=[13, 11][2(2)+4(3)+3(-1),\ 2(-2)+4(3)+3(1)] = [13,\ 11]

Row 2: [−1(2)+3(3)+2(−1), −1(−2)+3(3)+2(1)]=[5, 13][-1(2)+3(3)+2(-1),\ -1(-2)+3(3)+2(1)] = [5,\ 13]

AB=[1311513]AB=\begin{bmatrix}13&11\\5&13\end{bmatrix}

Step 2 — compute (AB)C (2×22\times2 times 2×22\times2):

Row 1: [13(3)+11(1), 13(1)+11(3)]=[50, 46][13(3)+11(1),\ 13(1)+11(3)] = [50,\ 46]

Row 2: [5(3)+13(1), 5(1)+13(3)]=[28, 44][5(3)+13(1),\ 5(1)+13(3)] = [28,\ 44]

(AB)C=[50462844](AB)C=\begin{bmatrix}50&46\\28&44\end{bmatrix}

Step 3 — compute BC (3×23\times2 times 2×22\times2):

Row 1: [2(3)+(−2)(1), 2(1)+(−2)(3)]=[4, −4][2(3)+(-2)(1),\ 2(1)+(-2)(3)] = [4,\ -4]

Row 2: [3(3)+3(1), 3(1)+3(3)]=[12, 12][3(3)+3(1),\ 3(1)+3(3)] = [12,\ 12]

Row 3: [−1(3)+1(1), −1(1)+1(3)]=[−2, 2][-1(3)+1(1),\ -1(1)+1(3)] = [-2,\ 2] …

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