Determine x + y if \(\begin{pmatrix} 2 & -3 \\ -1 & 4 \end{pmatrix}\) \(\begin{pmatrix} x \\ y \end{pmatrix}\) = \(\begin{pmatrix}-1 \\ 8 \end{pmatrix}\)
A
3
B
4
C
7
D
12
Verified Correct Answer: C.
Step-by-Step Mathematical Solution
\(\begin{pmatrix} 2 & -3 \\ -1 & 4 \end{pmatrix} \begin{pmatrix} x \\ y \end{pmatrix} = \begin{pmatrix} -1 \\ 8 \end{pmatrix}\)
\(\begin{pmatrix} 2x - 3y \\ -x + 4y \end{pmatrix} = \begin{pmatrix} -1 \\ 8 \end{pmatrix}\)
\(2x - 3y = -1 ... (i)\)
\(-x + 4y = 8 ... (ii)\)
From (ii), x = 4y - 8.
\(2(4y - 8) - 3y = -1 \implies 8y - 16 - 3y = -1\)
\(5y = -1 + 16 = 15 \implies y = 3\)
\(x = 4(3) - 8 = 12 - 8 = 4\)
\(\therefore x + y = 3 + 4 = 7\)
Source: Myschool Past Questions Archive (Record #30820)