Matrices & Determinants
System of linear equations — matrix method
Grade Class 12

Question:

For a real number &alpha;, if the system <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="[
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Step-by-Step Solution

Key Concept: For a system of linear equations AX = B to have infinitely many solutions, the determinant of the coefficient matrix must be zero, and the augmented matrix must have a rank less than the number of variables.
The system is AX = B where A = [[1, &alpha;, &alpha;^2], [&alpha;, 1, &alpha;], [&alpha;^2, &alpha;, 1]]. For infinitely many solutions, det(A) = 0. det(A) = 1(1 - &alpha;^2) - &alpha;(&alpha; - &alpha;^3) + &alpha;^2(&alpha;^2 - &alpha;^2) = 1 - &alpha;^2 - &alpha;^2 + &alpha;^4 = &alpha;^4 - 2&alpha;^2 + 1 = (&alpha;^2 - 1)^2 = 0. Thus &alpha;^2 = 1, so &alpha; = 1 or &alpha; = -1. If &alpha; = 1, the system becomes x + y + z = -1, x + y + z = 1, x + y + z = -1, which is inconsistent. If &alpha; = -1, the system becomes x - y + z = -1, -x + y - z = 1, x - y + z = -1, which simplifies to x - y + z = -1. This has infinitely many solutions. For &alpha; = -1, 1 + &alpha; + &alpha;^2 = 1 - 1 + 1 = 1.
Correct Answer: 1

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