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JEE Main 202228 JulyEvening Shift
Exam
JEE Main
Year
2022
Date
28 July
Shift
Evening Shift
Subject
Mathematics
Chapter
Quadratic Equation

JEE Main 2022 · 28 July · Evening Shift · Mathematics · Quadratic Equation

Let α, β be the roots of the equation x ^ 2 - 2 x + 6 = 0 and 1 α ^...

Let α,β\alpha, \beta be the roots of the equation x2−2x+6=0{x}^{2} - \sqrt{2}x + \sqrt{6} = 0 and 1α2+1,1β2+1\dfrac{1}{{\alpha}^{2}} + 1, \dfrac{1}{{\beta}^{2}} + 1 be the roots of the equation x2+ax+b=0{x}^{2} + ax + b = 0. Then the roots of the equation x2−(a+b−2)x+(a+b+2)=0{x}^{2} - \left(a + b - 2\right)x + \left(a + b + 2\right) = 0 are :

Options

  1. Anon-real complex numbers
  2. Breal and both negative
  3. Creal and both positive
  4. Dreal and exactly one of them is positive

Correct answer

B. real and both negative

Step-by-step solution

Given α,β\alpha, \beta be the roots of the equation x2−2x+6=0{x}^{2} - \sqrt{2}x + \sqrt{6} = 0

So sum of roots will be α+β=2\alpha + \beta = \sqrt{2} and product of roots will be αβ=6\alpha\beta = \sqrt{6}

And also given 1α2+1\dfrac{1}{{\alpha}^{2}} + 1 and 1β2+1\dfrac{1}{{\beta}^{2}} + 1 are roots of x2+ax+b=0{x}^{2} + ax + b = 0

So sum of roots will be −a=1α2+1+1β2+1- a = \dfrac{1}{{\alpha}^{2}} + 1 + \dfrac{1}{{\beta}^{2}} + 1

⇒a=−1α2−1β2−2.....(1)⇒ a = \dfrac{- 1}{{\alpha}^{2}} - \dfrac{1}{{\beta}^{2}} - 2 . . . . . \left(1\right)

And similarly product of roots will be,

b=1α2+1β2+1+1α2β2....(2)b = \dfrac{1}{{\alpha}^{2}} + \dfrac{1}{{\beta}^{2}} + 1 + \dfrac{1}{{\alpha}^{2}{\beta}^{2}} . . . . \left(2\right)

Now adding equation \left(1\right) \text{ and } \left(2\right)weget,</p><p>we get,</p><p>a + b = \dfrac{1}{{\left(\alpha\beta\right)}^{2}} - 1 = \dfrac{1}{6} - 1 = - \dfrac{5}{6}asαβ=6{as \alpha\beta = \sqrt{6}}

Now putting the value of a+ba + b in x2−(a+b−2)x+(a+b+2)=0{x}^{2} - \left(a + b - 2\right)x + \left(a + b + 2\right) = 0

⇒x2−(−56−2)x+(2−56)=0⇒ {x}^{2} - \left(- \dfrac{5}{6} - 2\right)x + \left(2 - \dfrac{5}{6}\right) = 0

⇒6x2+17x+7=0⇒ 6{x}^{2} + 17x + 7 = 0

⇒ x = - \dfrac{7}{3}, x = - \dfrac{1}{2} are the roots, both roots are real and negative.

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