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

If y = y(x) is the solution of the differential equation

${{dy} \over {dx}}$ + (tan x) y = sin x, $0 \le x \le {\pi \over 3}$, with y(0) = 0, then $y\left( {{\pi \over 4}} \right)$ equal to :

  1. A ${1 \over 2}$log<sub>e</sub> 2
  2. B $\left( {{1 \over {2\sqrt 2 }}} \right)$ log<sub>e</sub> 2 Correct answer
  3. C log<sub>e</sub> 2
  4. D ${1 \over 4}$ log<sub>e</sub> 2

Solution

Integrating Factor $= {e^{\int {\tan x\,dx} }} = {e^{\ln (\sec x)}} = \sec x$<br><br>$y\sec x = \int {(\sin x)\sec x\,dx = \ln (\sec x) + C}$<br><br>$y(0) = 0 \Rightarrow C = 0$<br><br>$\therefore$ $y = \cos x\ln |\sec x|$<br><br>$$y\left( {{\pi \over 4}} \right) = {1 \over {\sqrt 2 }}\ln \left( {\sqrt 2 } \right) = {1 \over {2\sqrt 2 }}\ln 2$$

About this question

Subject: Mathematics · Chapter: Differential Equations · Topic: Order and Degree

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