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Mathematics

If A = [2639]\begin{bmatrix} 2 & 6 \ 3 & 9 \end{bmatrix}, B = [3xy2]\begin{bmatrix} 3 & x \ y & 2 \end{bmatrix} and AB = 0, find the values of x and y.

Matrices

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Answer

Given,

A = [2639]\begin{bmatrix} 2 & 6 \ 3 & 9 \end{bmatrix}, B = [3xy2]\begin{bmatrix} 3 & x \ y & 2 \end{bmatrix}

AB = 0

[2639]×[3xy2]=[0000][(2)(3)+6y2x+(6)(2)(3)(3)+9y3x+(9)(2)]=[0000][6+6y2x+129+9y3x+18]=[0000]\Rightarrow \begin{bmatrix} 2 & 6 \ 3 & 9 \end{bmatrix} \times \begin{bmatrix} 3 & x \ y & 2 \end{bmatrix} = \begin{bmatrix} 0 & 0 \ 0 & 0 \end{bmatrix} \\[1em] \Rightarrow \begin{bmatrix} (2)(3) + 6y & 2x + (6)(2) \ (3)(3) + 9y & 3x + (9)(2) \end{bmatrix} = \begin{bmatrix} 0 & 0 \ 0 & 0 \end{bmatrix} \\[1em] \Rightarrow \begin{bmatrix} 6 + 6y & 2x + 12 \ 9 + 9y & 3x + 18 \end{bmatrix} = \begin{bmatrix} 0 & 0 \ 0 & 0 \end{bmatrix}

Solve for x and y:

∴ 6 + 6y = 0

⇒ 6y = -6

⇒ y = 66\dfrac{-6}{6}

⇒ y = -1.

∴ 2x + 12 = 0

⇒ 2x = -12

⇒ x = 122\dfrac{-12}{2}

⇒ x = -6.

Hence, x = -6 and y = -1.

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