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JEE Main 201912 JanuaryMorning Shift
Exam
JEE Main
Year
2019
Date
12 January
Shift
Morning Shift
Subject
Mathematics
Chapter
Quadratic Equation

JEE Main 2019 · 12 January · Morning Shift · Mathematics · Quadratic Equation

If λ be the ratio of the roots of the quadratic equation in x, 3 m...

If λ\lambda be the ratio of the roots of the quadratic equation in x,3m2x2+m(m−4)x+2=0x, 3{m}^{2}{x}^{2} + m\left(m - 4\right)x + 2 = 0, then the least value of mm for which λ+1λ=1\lambda + \dfrac{1}{\lambda} = 1, is :

Options

  1. A2−32 - \sqrt{3}
  2. B−2+2- 2 + \sqrt{2}
  3. C4−234 - 2\sqrt{3}
  4. D4−324 - 3\sqrt{2}

Correct answer

D. 4−324 - 3\sqrt{2}

Step-by-step solution

Let αand\alpha and\betaaretherootsofgivenequationare the roots of given equation3{m}^{2}{x}^{2} + m\left(m - 4\right)x + 2 = 0.</p><p>.</p><p>∴ \alpha + \beta = - \dfrac{m\left(m - 4\right)}{3{m}^{2}} \text{andand} \alpha\beta = \dfrac{2}{3{m}^{2}} . . . . . . . . . \left(i\right)</p><p>Given</p><p>Given\dfrac{\alpha}{\beta} = \lambda \text{andand} \lambda + \dfrac{1}{\lambda} = 1</p><p></p><p>⇒ \dfrac{\alpha}{\beta} + \dfrac{\beta}{\alpha} = 1</p><p></p><p>⇒ {\left(\alpha + \beta\right)}^{2} = 3\alpha\beta</p><p>Usingequation(i)</p><p>Using equation \left(i\right), we get

(−m(m−4)3m2)2=3×23m2{\left(\dfrac{- m\left(m - 4\right)}{3{m}^{2}}\right)}^{2} = \dfrac{3 \times 2}{3{m}^{2}}

⇒m=4±32⇒ m = 4 \pm 3\sqrt{2}

Hence, the least value of mm is 4−324 - 3\sqrt{2}.

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