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JEE Advanced 2013Paper 1
Exam
JEE Advanced
Year
2013
Paper
Paper 1
Subject
Mathematics
Chapter
Differential Equations

JEE Advanced 2013 · Paper 1 · Mathematics · Differential Equations

A curve passes through the point (1 , (π)/(6)) . Let the slope of...

A curve passes through the point (1,π6)\left(1 , \dfrac{\pi}{6}\right). Let the slope of the curve at each point (x,y)\left(\text{x} , \text{y}\right) be yx+sec(yx),x>0\dfrac{\text{y}}{\text{x}} + \text{sec} \left(\dfrac{\text{y}}{\text{x}}\right) , x > 0. Then the equation of the curve is

Options

  1. A\text\sin \left(\dfrac{\text{y}}{\text{x}}\right) = \text{log x} + \dfrac{1}{2}
  2. Bcosec(yx)=log x+2\text{cosec} \left(\dfrac{\text{y}}{\text{x}}\right) = \text{log x} + 2
  3. Csec(2yx)=log x+2\text{sec} \left(\dfrac{\text{2y}}{\text{x}}\right) = \text{log x} + 2
  4. D\text\cos \left(\dfrac{\text{2y}}{\text{x}}\right) = \text{log x} + \dfrac{1}{2}

Correct answer

A. \text\sin \left(\dfrac{\text{y}}{\text{x}}\right) = \text{log x} + \dfrac{1}{2}

Step-by-step solution

Given slope at (x, y) is

dydx=yx+sec(yx)\dfrac{\text{dy}}{\text{dx}} = \dfrac{\text{y}}{\text{x}} + \text{sec}\left(\dfrac{y}{x}\right)......\left(i\right)<br/>let<br />let\dfrac{\text{y}}{\text{x}} = \text{t} ⇒ \text{y} = \text{xt}</p><p>Differentiatingthiswrt</p><p>Differentiating this wrtx</p><p></p><p>⇒ \dfrac{\text{dy}}{\text{dx}} = \text{t} + \text{x}\dfrac{\text{dt}}{\text{dx}}......(ii)......\left(ii\right)

Now using equation \left(i\right)and(ii)and \left(ii\right), we have
t+xdtdx=t+sec(t)\text{t} + \text{x} \dfrac{\text{dt}}{\text{dx}} = \text{t} + \text{sec} \left(\text{t}\right)

⇒dtsec⁡(t)=dxx⇒ \dfrac{dt}{\sec \left(t\right)} = \dfrac{dx}{x}

Integrating both sides.
∫cos⁡(t)dt=∫1xdx∫ \cos \left(t\right)\text{dt} = ∫ \dfrac{1}{\text{x}}\text{dx}
sin⁡(t)=ln⁡(x)+c\sin \left(t\right) = \ln \left(x\right) + \text{c}

Now Put value of tt in above equation.
\text\sin\left(\dfrac{y}{x}\right) = ln\left(x\right) + \text{c}.....(iii).....\left(iii\right)
Given that curve passes through \left(1\text{,}\dfrac{\pi}{6}\right)soputinaboveequation.<br/>so put in above equation.<br />\text\sin\left(\dfrac{\pi}{6}\right) = \ln \left(1\right) + \text{c} ⇒ \text{c} = \dfrac{1}{2}</p><p>Now,putthevalueof</p><p>Now, put the value ofcinequation(iii)in equation \left(iii\right)
\text\sin\left(\dfrac{y}{x}\right) = \ln \left(x\right) + \dfrac{1}{2}

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