Matrices & Determinants
Determinants
Grade Class 12

Question:

Let α, β, γ be the real roots of the equation, x³ + ax² + bx + c = 0, (a, b, c ∈ R and a, b ≠ 0). If the system of equations (in, u, v, w) given by αu + βv + γw = 0, βu + γv + αw = 0; γu + αv + βw = 0 has non-trivial solution, then the value of a²/b is
(1) 5
(2) 3
(3) 1
(4) 0

Step-by-Step Solution

Key Concept: For a system of linear equations to have a non-trivial solution, the determinant of the coefficient matrix must be zero. The determinant is a circulant determinant involving roots of the cubic equation.
The system of equations is: \alpha u + \beta v + \gamma w = 0, \beta u + \gamma v + \alpha w = 0, \gamma u + \alpha v + \beta w = 0. For a non-trivial solution, the determinant of the coefficient matrix must be zero: |\alpha \beta \gamma; \beta \gamma \alpha; \gamma \alpha \beta| = 0. This simplifies to -(\alpha^3 + \beta^3 + \gamma^3 - 3\alpha\beta\gamma) = 0, which means \alpha^3 + \beta^3 + \gamma^3 = 3\alpha\beta\gamma. Since \alpha, \beta, \gamma are roots of x^3 + ax^2 + bx + c = 0, we have \alpha+\beta+\gamma = -a, \alpha\beta+\beta\gamma+\gamma\alpha = b, \alpha\beta\gamma = -c. Using the identity \alpha^3 + \beta^3 + \gamma^3 - 3\alpha\beta\gamma = (\alpha+\beta+\gamma)((\alpha+\beta+\gamma)^2 - 3(\alpha\beta+\beta\gamma+\gamma\alpha)), we get -a(a^2 - 3b) = 0. Since a \neq 0, we have a^2 - 3b = 0, so a^2/b = 3.
Correct Answer: 2

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