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JEE Main 20149 AprilOnline
Exam
JEE Main
Year
2014
Date
9 April
Mode
Online
Subject
Mathematics
Chapter
Quadratic Equation

JEE Main 2014 · 9 April · Online · Mathematics · Quadratic Equation

If 1 α , 1 β are the roots of the equation a x ^ 2 + bx + 1 = 0, (a...

If 1α,1β\dfrac{1}{\sqrt{\alpha}}, \dfrac{1}{\sqrt{\beta}} are the roots of the equation ax2+bx+1=0,(a≠0,a,b∈R)a{x}^{2} + bx + 1 = 0, \left(a \ne 0, a, b \in R\right), then the equation x(x+b3)+(a3−3abx)=0x\left(x + {b}^{3}\right) + \left({a}^{3} - 3abx\right) = 0 has roots:

Options

  1. Aαβ\sqrt{\alpha\beta} and αβ\alpha\beta
  2. Bα−32{\alpha}^{- \dfrac{3}{2}} and β−32{\beta}^{- \dfrac{3}{2}}
  3. Cαβ12\alpha\beta\dfrac{1}{2} and α12β\alpha\dfrac{1}{2}\beta
  4. Dα32\alpha\dfrac{3}{2} and β32\beta\dfrac{3}{2}

Correct answer

D. α32\alpha\dfrac{3}{2} and β32\beta\dfrac{3}{2}

Step-by-step solution

a{x}^{2} + bx + 1 = 0hasrootshas roots\dfrac{1}{\sqrt{\alpha}}, \dfrac{1}{\sqrt{\beta}}<br/><br/><br /><br />⇒ \dfrac{1}{\sqrt{\alpha}} + \dfrac{1}{\sqrt{\beta}} = - \dfrac{b}{a} . . . \left(1\right)<br/><br/><br /><br />\dfrac{1}{\sqrt{\alpha}\sqrt{\beta}} = \dfrac{1}{a} . . . \left(2\right)<br/><br/><br /><br />x\left(x + {b}^{3}\right) + \left({a}^{3} - 3abx\right) = 0<br/><br/><br /><br />⇒ {x}^{2} + \left({b}^{3} - 3ab\right)x + {a}^{3} = 0</p><p>Multiplyanddividewith</p><p>Multiply and divide with{a}^{3},weget<br/><br/>, we get<br /><br />⇒ {x}^{2} + {a}^{3}\left\{\left(\dfrac{{b}^{3}}{{a}^{3}}\right) - 3\dfrac{1}{a}\left(\dfrac{b}{a}\right)\right\}x + {a}^{3} = 0<br/>Fromequations(1) and (2)<br />From equations \left(1\right) \text{ and } \left(2\right),
⇒x2+(αβ)3/2[−(α1/2+β1/2)3(αβ)3−31αβα+βαβ]x+(αβ)3/2=0⇒ {x}^{2} + {\left(\alpha\beta\right)}^{3 / 2}\left[- \dfrac{{\left({\alpha}^{1 / 2} + {\beta}^{1 / 2}\right)}^{3}}{{\left(\sqrt{\alpha\beta}\right)}^{3}} - 3\dfrac{1}{\sqrt{\alpha\beta}}\dfrac{\sqrt{\alpha} + \sqrt{\beta}}{\sqrt{\alpha\beta}}\right]x + {\left(\alpha\beta\right)}^{3 / 2} = 0

⇒x2−(α3/2+β3/2)x+α3/2β3/2=0⇒ {x}^{2} - \left({\alpha}^{3 / 2} + {\beta}^{3 / 2}\right)x + {\alpha}^{3 / 2}{\beta}^{3 / 2} = 0

Roots are α32andβ32\alpha\dfrac{3}{2}\text{and}\beta\dfrac{3}{2}.

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