Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Differential Equations question

2024 · 31 Jan · Shift 1 · Q46
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Differential Equations
  5. /2024 · 31 Jan · Shift 1 · Q46

Differential Equations question

2024 · 31 Jan · Shift 1 · Q46

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
The solution curve of the differential equation ydxdy=x(log⁡ex−log⁡ey+1),x>0,y>0y \frac{d x}{d y}=x\left(\log _e x-\log _e y+1\right), x\gt 0, y\gt 0ydydx​=x(loge​x−loge​y+1),x>0,y>0 passing through the point (e,1)(e, 1)(e,1) is
  1. A
    ∣log⁡eyx∣=y2\left|\log _e \frac{y}{x}\right|=y^2​loge​xy​​=y2
  2. B
    ∣log⁡eyx∣=x\left|\log _e \frac{y}{x}\right|=x​loge​xy​​=x
  3. C
    ∣log⁡exy∣=y\left|\log _e \frac{x}{y}\right|=y​loge​yx​​=y
  4. D
    2∣log⁡exy∣=y+12\left|\log _e \frac{x}{y}\right|=y+12​loge​yx​​=y+1
View written solutionFree

Correct answer: C

  1. Given differential equation

ydxdy=x(ln⁡x−ln⁡y+1),x>0, y>0y\frac{dx}{dy}=x\left(\ln x-\ln y+1\right), \qquad x>0,\ y>0ydydx​=x(lnx−lny+1),x>0, y>0

We need the solution curve passing through (x,y)=(e,1)(x,y)=(e,1)(x,y)=(e,1).


  1. Simplify the logarithmic expression

Using ln⁡x−ln⁡y=ln⁡xy,\ln x-\ln y=\ln\frac{x}{y},lnx−lny=lnyx​, we get

ydxdy=x(ln⁡xy+1).y\frac{dx}{dy}=x\left(\ln\frac{x}{y}+1\right).ydydx​=x(lnyx​+1).

Divide by xxx:

yxdxdy=ln⁡xy+1.\frac{y}{x}\frac{dx}{dy}=\ln\frac{x}{y}+1.xy​dydx​=lnyx​+1.

Now notice that a substitution involving xy\dfrac{x}{y}yx​ will help.


  1. Use substitution

Let

u=ln⁡xy=ln⁡x−ln⁡y.u=\ln\frac{x}{y}=\ln x-\ln y.u=lnyx​=lnx−lny.

Differentiate with respect to yyy:

dudy=1xdxdy−1y.\frac{du}{dy}=\frac{1}{x}\frac{dx}{dy}-\frac{1}{y}.dydu​=x1​dydx​−y1​.

Multiply by yyy:

ydudy=yxdxdy−1.y\frac{du}{dy}=\frac{y}{x}\frac{dx}{dy}-1.ydydu​=xy​dydx​−1.

From the differential equation,

yxdxdy=u+1.\frac{y}{x}\frac{dx}{dy}=u+1.xy​dydx​=u+1.

So,

ydudy=(u+1)−1=u.y\frac{du}{dy}=(u+1)-1=u.ydydu​=(u+1)−1=u.

Thus the equation reduces to

ydudy=u.y\frac{du}{dy}=u.ydydu​=u.


  1. Solve the separable equation

ydudy=u⇒duu=dyy.y\frac{du}{dy}=u \quad \Rightarrow \quad \frac{du}{u}=\frac{dy}{y}.ydydu​=u⇒udu​=ydy​.

Integrating,

ln⁡∣u∣=ln⁡y+C.\ln|u|=\ln y + C.ln∣u∣=lny+C.

Hence,

∣u∣=Cy,|u|=Cy,∣u∣=Cy, where C>0C>0C>0 is a constant.

Substitute back u=ln⁡xyu=\ln\dfrac{x}{y}u=lnyx​:

∣ln⁡xy∣=Cy.\left|\ln\frac{x}{y}\right|=Cy.​lnyx​​=Cy.


  1. Use the point (e,1)(e,1)(e,1)

At x=e,y=1x=e, y=1x=e,y=1,

∣ln⁡e1∣=C(1).\left|\ln\frac{e}{1}\right|=C(1).​ln1e​​=C(1).

Since ln⁡e=1\ln e=1lne=1,

∣1∣=C⇒C=1.|1|=C \Rightarrow C=1.∣1∣=C⇒C=1.

Therefore,

∣ln⁡xy∣=y.\boxed{\left|\ln\frac{x}{y}\right|=y}.​lnyx​​=y​.


  1. Match with options

This is exactly Option C:

∣log⁡exy∣=y.\boxed{\left|\log_e\frac{x}{y}\right|=y}.​loge​yx​​=y​.

So the correct answer is C.

PreviousNext

More from Differential Equations

  • The temperature T(t) of a body at time t=0 is 160∘F and it decreases continuously as per the differential equation dtdT​=−K(T−80), where K is a positive constant. If T(15)=120∘F, then…2024 · MCQ
  • Let y=y(x) be the solution of the differential equation sec2xdx+(e2ytan2x+tanx)dy=0,0<x<2π​,y(π/4)=0. If y(π/6)=α, then e8α is equal to ​…2024 · Numerical
  • The area enclosed by the closed curve C given by the differential equation dxdy​+y−2x+a​=0,y(1)=0 is 4π. Let P and Q be the points of intersection of the curve C and the y-axis. If…2023 · MCQ
  • If y=y(x) is the solution curve of the differential equation dxdy​+ytanx=xsecx,0≤x≤3π​,y(0)=1, then y(6π​) is equal to2023 · MCQ
  • Let αx=exp(xβyγ) be the solution of the differential equation 2x2y dy−(1−xy2)dx=0,x>0,y(2)=loge​2​. Then α+β−γ equals :2023 · MCQ
  • Let y=y(x) be a solution of the differential equation (xcosx)dy+(xysinx+ycosx−1)dx=0,0<x<2π​. If 3π​y(3π​)=3​, then ​6π​y′′(6π​)+2y′(6π​)​…2023 · Numerical
  • If the solution curve f(x,y)=0 of the differential equation (1+loge​x)dydx​−xloge​x=ey,x>0, passes through the points (1,0) and (α,2), then αα is equal to :2023 · MCQ
  • If the solution curve of the differential equation (y−2loge​x)dx+(xloge​x2)dy=0,x>1 passes through the points (e,34​) and (e4,α), then α…2023 · Numerical