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JEE Main 202224 JuneEvening Shift
Exam
JEE Main
Year
2022
Date
24 June
Shift
Evening Shift
Subject
Mathematics
Chapter
Quadratic Equation

JEE Main 2022 · 24 June · Evening Shift · Mathematics · Quadratic Equation

The sum of all real roots of equation ( e ^ 2x - 4)(6 e ^ 2x - 5 e...

The sum of all real roots of equation (e2x−4)(6e2x−5ex+1)=0\left({e}^{2x} - 4\right)\left(6{e}^{2x} - 5{e}^{x} + 1\right) = 0 is

Options

  1. Aln⁡4\ln 4
  2. B−ln⁡3- \ln 3
  3. Cln⁡3\ln 3
  4. Dln⁡5\ln 5

Correct answer

B. −ln⁡3- \ln 3

Step-by-step solution

Let ex=t{e}^{x} = t so, e2x=t2{e}^{2x} = {t}^{2}

So, the given equation becomes, \left({t}^{2} - 4\right)\left(6{t}^{2} - 5t + 1\right) = 0</p><p></p><p>{t}^{2} - 4 = 0(or)(or)6{t}^{2} - 5t + 1 = 0</p><p></p><p>{t}^{2} = 4(or)(or)6{t}^{2} - 3t - 2t + 1 = 0</p><p></p><p>t = \pm 2(or)(or)3t\left(2t - 1\right) - 1\left(2t - 1\right) = 0</p><p></p><p>t = \pm 2(or)(3t−1)(2t−1)=0(or) \left(3t - 1\right)\left(2t - 1\right) = 0

t=±2t = \pm 2 (or) t=13t = \dfrac{1}{3} (or) t=12t = \dfrac{1}{2}

But, ex=t>0(∵ex>0;∀x∈R){e}^{x} = t > 0\left(∵ {e}^{x} > 0; \forall x \in R\right)

∴∴ Only possible values for tt are 2,13,122, \dfrac{1}{3}, \dfrac{1}{2}

When t=2,ex=2⇒x=log⁡e(2)t = 2, {e}^{x} = 2 ⇒ x = \log_{e}\left(2\right) (or) ln⁡2\ln 2

When t=13,ex=13⇒x=log⁡e(13)t = \dfrac{1}{3}, {e}^{x} = \dfrac{1}{3} ⇒ x = \log_{e}\left(\dfrac{1}{3}\right) or −ln⁡3- \ln 3

When t=12,ex=12⇒x=log⁡e(12)t = \dfrac{1}{2}, {e}^{x} = \dfrac{1}{2} ⇒ x = \log_{e}\left(\dfrac{1}{2}\right) or −ln⁡2- \ln 2

∴∴ The sum of roots of given equation =ln⁡2−ln⁡3−ln⁡2=−ln⁡3= \ln 2 - \ln 3 - \ln 2 = - \ln 3.

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