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Differential Equations question

2025 · Shift 2 · Q25
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  5. /2025 · Shift 2 · Q25

Differential Equations question

2025 · Shift 2 · Q25

JEE AdvancedMathematicsDifferential EquationsNumerical+4 / −1
Let y(x)y(x)y(x) be the solution of the differential equation x2dydx+xy=x2+y2,x>1ex^2 \frac{d y}{d x}+x y=x^2+y^2, \quad x\gt \frac{1}{e}x2dxdy​+xy=x2+y2,x>e1​ satisfying y(1)=0y(1)=0y(1)=0. Then the value of 2(y(e))2y(e2)2 \frac{(y(e))^2}{y\left(e^2\right)}2y(e2)(y(e))2​ is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 0.7TO0.8

  1. Given differential equation

x2dydx+xy=x2+y2,x>1e,y(1)=0x^2\frac{dy}{dx}+xy=x^2+y^2, \qquad x>\frac1e, \qquad y(1)=0x2dxdy​+xy=x2+y2,x>e1​,y(1)=0

We need to find

2(y(e))2y(e2).2\frac{(y(e))^2}{y(e^2)}.2y(e2)(y(e))2​.


  1. Rewrite the equation

Divide by x2x^2x2:

dydx+1xy=1+(yx)2.\frac{dy}{dx}+\frac{1}{x}y=1+\left(\frac{y}{x}\right)^2.dxdy​+x1​y=1+(xy​)2.

This suggests the substitution

v=yx⇒y=vx.v=\frac{y}{x} \quad \Rightarrow \quad y=vx.v=xy​⇒y=vx.

Then

dydx=v+xdvdx.\frac{dy}{dx}=v+x\frac{dv}{dx}.dxdy​=v+xdxdv​.

Substitute into the differential equation:

v+xdvdx+v=1+v2.v+x\frac{dv}{dx}+v=1+v^2.v+xdxdv​+v=1+v2.

So,

xdvdx=v2−2v+1=(v−1)2.x\frac{dv}{dx}=v^2-2v+1=(v-1)^2.xdxdv​=v2−2v+1=(v−1)2.

Hence,

dv(v−1)2=dxx.\frac{dv}{(v-1)^2}=\frac{dx}{x}.(v−1)2dv​=xdx​.


  1. Integrate

Integrating both sides,

∫dv(v−1)2=∫dxx.\int \frac{dv}{(v-1)^2}=\int \frac{dx}{x}.∫(v−1)2dv​=∫xdx​.

Now,

∫(v−1)−2dv=−1v−1.\int (v-1)^{-2}dv=-\frac{1}{v-1}.∫(v−1)−2dv=−v−11​.

Therefore,

−1v−1=ln⁡x+C.-\frac{1}{v-1}=\ln x + C.−v−11​=lnx+C.

Use the initial condition y(1)=0y(1)=0y(1)=0. At x=1x=1x=1,

v(1)=y(1)1=0.v(1)=\frac{y(1)}{1}=0.v(1)=1y(1)​=0.

So,

−10−1=ln⁡1+C⇒1=0+C⇒C=1.-\frac{1}{0-1}=\ln 1 + C \Rightarrow 1=0+C \Rightarrow C=1.−0−11​=ln1+C⇒1=0+C⇒C=1.

Thus,

−1v−1=ln⁡x+1.-\frac{1}{v-1}=\ln x+1.−v−11​=lnx+1.

Rearrange:

1v−1=−(ln⁡x+1)\frac{1}{v-1}=-(\ln x+1)v−11​=−(lnx+1)

v−1=−1ln⁡x+1v-1=-\frac{1}{\ln x+1}v−1=−lnx+11​

v=1−1ln⁡x+1=ln⁡xln⁡x+1.v=1-\frac{1}{\ln x+1}=\frac{\ln x}{\ln x+1}.v=1−lnx+11​=lnx+1lnx​.

Since v=yxv=\dfrac{y}{x}v=xy​,

y=x⋅ln⁡xln⁡x+1.y=x\cdot \frac{\ln x}{\ln x+1}.y=x⋅lnx+1lnx​.


  1. Compute y(e)y(e)y(e) and y(e2)y(e^2)y(e2)

For x=ex=ex=e,

ln⁡e=1\ln e=1lne=1

so

y(e)=e⋅12=e2.y(e)=e\cdot \frac{1}{2}=\frac{e}{2}.y(e)=e⋅21​=2e​.

For x=e2x=e^2x=e2,

ln⁡(e2)=2\ln(e^2)=2ln(e2)=2

so

y(e2)=e2⋅23=2e23.y(e^2)=e^2\cdot \frac{2}{3}=\frac{2e^2}{3}.y(e2)=e2⋅32​=32e2​.


  1. Evaluate the required expression

2(y(e))2y(e2)=2⋅(e2)22e23.2\frac{(y(e))^2}{y(e^2)}=2\cdot \frac{\left(\frac e2\right)^2}{\frac{2e^2}{3}}.2y(e2)(y(e))2​=2⋅32e2​(2e​)2​.

Now,

(e2)2=e24.\left(\frac e2\right)^2=\frac{e^2}{4}.(2e​)2=4e2​.

Hence,

2⋅e2/42e2/3=2⋅e24⋅32e2.2\cdot \frac{e^2/4}{2e^2/3}=2\cdot \frac{e^2}{4}\cdot \frac{3}{2e^2}.2⋅2e2/3e2/4​=2⋅4e2​⋅2e23​.

Cancel e2e^2e2:

=2⋅38=34.=2\cdot \frac{3}{8}=\frac{3}{4}.=2⋅83​=43​.

So the required value is

\boxed{\frac34}}

Since this is an integer-type question with range-style stored answer, numerically

34=0.75.\frac34=0.75.43​=0.75.


  1. Comparison with stored correct answer

Stored correct answer: 0.7TO0.8

Our derived value is 0.750.750.75, which lies in this range. So it agrees.

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