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If M=[2−13]M = \begin{bmatrix} 2 \\ -1 \\ 3 \end{bmatrix} and N=[71−4]N = [7 \quad 1 \quad -4], then (MN)T(MN)^T will be equal to:

Solution

✅ Correct Option: 4

Given:

M=[2−13]M = \begin{bmatrix} 2 \\ -1 \\ 3 \end{bmatrix} (column vector, 3×1)

N=[71−4]N = [7 \quad 1 \quad -4] (row vector, 1×3)

To find (MN)T(MN)^T, first multiply MM and NN.


Multiplying a (3×1) matrix with a (1×3) matrix gives a (3×3) matrix.

Each element in MM multiplies with the entire row NN:

MN=[2−13][71−4]MN = \begin{bmatrix} 2 \\ -1 \\ 3 \end{bmatrix} \begin{bmatrix} 7 & 1 & -4 \end{bmatrix}

Row 1: 2×7=142 × 7 = 14, 2×1=22 × 1 = 2, 2×(−4)=−82 × (-4) = -8

Row 2: (−1)×7=−7(-1) × 7 = -7, (−1)×1=−1(-1) × 1 = -1, (−1)×(−4)=4(-1) × (-4) = 4

Row 3: 3×7=213 × 7 = 21, 3×1=33 × 1 = 3, 3×(−4)=−123 × (-4) = -12

MN=[142−8−7−14213−12]MN = \begin{bmatrix} 14 & 2 & -8 \\ -7 & -1 & 4 \\ 21 & 3 & -12 \end{bmatrix}


The transpose swaps rows and columns.

Row 1 becomes Column 1, Row 2 becomes Column 2, Row 3 becomes Column 3:

(MN)T=[14−7212−13−84−12](MN)^T = \begin{bmatrix} 14 & -7 & 21 \\ 2 & -1 & 3 \\ -8 & 4 & -12 \end{bmatrix}

Therefore, (MN)T=[14−7212−13−84−12](MN)^T = \begin{bmatrix} 14 & -7 & 21 \\ 2 & -1 & 3 \\ -8 & 4 & -12 \end{bmatrix}

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