Medium MCQ +4 / -1 PYQ · JEE Mains 2021

The sum of the roots of the equation

$x + 1 - 2{\log _2}(3 + {2^x}) + 2{\log _4}(10 - {2^{ - x}}) = 0$, is :

  1. A log<sub>2</sub> 14
  2. B log<sub>2</sub> 11 Correct answer
  3. C log<sub>2</sub> 12
  4. D log<sub>2</sub> 13

Solution

$x + 1 - 2{\log _2}(3 + {2^x}) + 2{\log _4}(10 - {2^{ - x}}) = 0$<br><br>$${\log _2}({2^{x + 1}}) - {\log _2}{(3 + {2^x})^2} + {\log _2}(10 - {2^{ - x}}) = 0$$<br><br>$$lo{g_2}\left( {{{{2^{x + 1}}.(10 - {2^{ - x}})} \over {{{(3 + {2^x})}^2}}}} \right) = 0$$<br><br>${{2({{10.2}^{ - x}} - 1)} \over {{{(3 + {2^x})}^2}}} = 1$<br><br>$\Rightarrow {20.2^x} - 2 = 9 + {2^{2x}} + {6.2^x}$<br><br>$\therefore$ ${({2^x})^2} - 14({2^x}) + 11 = 0$<br><br>Roots are 2<sup>x<sub>1</sub></sup> &amp; 2<sup>x<sub>2</sub></sup><br><br>$\therefore$ 2<sup>x<sub>1</sub></sup> . 2<sup>x<sub>2</sub></sup> = 11<br><br>x<sub>1</sub> + x<sub>2</sub> = log<sub>2</sub>(11)

About this question

Subject: Mathematics · Chapter: Complex Numbers and Quadratic Equations · Topic: Complex Numbers and Argand Plane

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