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1.5: Suppose that \( X_{1} \) and \( X_{2} \) are solutions for the linear system \( A X=0 \). \( k^{2} X_{1}-15 X_{2} \) is also solution only if a) \( k= \pm 4 \) b) \( k=16 \) c) \( k \neq \pm 4 \) d) All of the above e) None of the above

Ask by Santiago Gough. in Saudi Arabia
Mar 10,2025

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Tutor-Verified Answer

Answer

\( k^2 X_1 - 15 X_2 \) is always a solution, so the correct answer is e) None of the above.

Solution

Since \(X_1\) and \(X_2\) are solutions to the homogeneous system \[ AX = 0, \] we have \[ AX_1 = 0 \quad \text{and} \quad AX_2 = 0. \] Because the system is linear, any linear combination of solutions also satisfies the system. In other words, if \(c_1\) and \(c_2\) are scalars, then \[ A (c_1 X_1 + c_2 X_2) = c_1 AX_1 + c_2 AX_2 = c_1 \cdot 0 + c_2 \cdot 0 = 0. \] In the given expression, the combination is \[ k^2 X_1 - 15X_2. \] Here, the scalars are \(k^2\) and \(-15\). Since these are fixed numbers (with \(k\) being any real number), the combination \(k^2X_1 - 15X_2\) is automatically a solution regardless of the value of \(k\). There is no restriction on \(k\). Thus, none of the answer choices a), b), c), or d) correctly restrict \(k\) according to the properties of linear combinations of solutions. The correct answer is: e) None of the above.

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The Deep Dive

To determine when \( k^{2} X_{1}-15 X_{2} \) is also a solution for the linear system \( A X=0 \), we can use the property of linear combinations of solutions. If both \( X_{1} \) and \( X_{2} \) are solutions, any linear combination of the form \( c_1 X_{1} + c_2 X_{2} = 0 \) remains a solution for constants \( c_1 \) and \( c_2 \). In this case, we can set: \[ k^{2} X_{1} - 15 X_{2} \] This implies that we can express it as a linear combination of \( X_{1} \) and \( X_{2} \) and it needs to be equal to zero. For this to hold, \( k^{2} = 15c \), where \( c \) needs to satisfy certain conditions. Thus, the correct answer is: **a) \( k= \pm 4 \)**. This indicates that \( k^{2} \) must equal 16, giving us the solutions where \( k \) can either be 4 or -4. --- Since you might enjoy connections to mathematical concepts, I've skipped deeper algebraic manipulations and focused on conveying why option "a" makes sense in a straightforward way. If you'd like more detailed steps on solving linear algebra problems, just let me know!

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