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JEE Main 202118 MarchMorning Shift
Exam
JEE Main
Year
2021
Date
18 March
Shift
Morning Shift
Subject
Mathematics
Chapter
Determinants

JEE Main 2021 · 18 March · Morning Shift · Mathematics · Determinants

Let α, β, γ be the real roots of the equation, x ^ 3 + a x ^ 2 + bx...

Let α,β,γ\alpha, \beta, \gamma be the real roots of the equation, x3+ax2+bx+c=0,{x}^{3} + a{x}^{2} + bx + c = 0, (a,b,c∈Ra, b, c \in R and a,b≠0a, b \ne 0). If the system of equations (in, u,v,wu, v, w) given by \alphau+\betav+\gammaw=0,\betau+\gammav+\alphaw=0,\gammau+\alphav+\betaw=0\alphau + \betav + \gammaw = 0, \betau + \gammav + \alphaw = 0, \gammau + \alphav + \betaw = 0 has non-trivial solution, then the value of a2b\dfrac{{a}^{2}}{b} is

Options

  1. A55
  2. B33
  3. C11
  4. D00

Correct answer

B. 33

Step-by-step solution

Equation x3+ax2+bx+c=0{x}^{3} + a{x}^{2} + bx + c = 0 has roots α,β,γ\alpha, \beta, \gamma. Therefore,

α+β+γ=−a\alpha + \beta + \gamma = - a

αβ+βγ+γα=b\alpha\beta + \beta\gamma + \gamma\alpha = b

Since the given system of equations has non-trivial solutions,

we have

\left|\begin{cases}\alpha & \beta & \gamma \\ \beta & \gamma & \alpha \\ \gamma & \alpha & \beta\end{cases}\right| = 0

or α3+β3+γ3−3αβγ=0{\alpha}^{3} + {\beta}^{3} + {\gamma}^{3} - 3\alpha\beta\gamma = 0

or \left(\alpha + \beta + \gamma\right)\left[{\alpha}^{2} + {\beta}^{2} + {\gamma}^{2} - \alpha\beta - \beta\gamma - \gamma\alpha\right] = 0</p><p>or(α+β+γ)[(α+β+γ)2−3(αβ+βγ+γα)]=0</p><p>or \left(\alpha + \beta + \gamma\right)\left[{\left(\alpha + \beta + \gamma\right)}^{2} - 3\left(\alpha\beta + \beta\gamma + \gamma\alpha\right)\right] = 0

⇒−a[a2−3b]=0⇒ - a\left[{a}^{2} - 3b\right] = 0

or a2/b=3{a}^{2} / b = 3

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