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Differential Equations question

2021 · 25 Feb · Shift 2 · Q48
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  5. /2021 · 25 Feb · Shift 2 · Q48

Differential Equations question

2021 · 25 Feb · Shift 2 · Q48

JEE MainMathematicsDifferential EquationsNumerical+4 / −1
If the curve, y = y(x) represented by the solution of the differential equation (2xy2 −-− y)dx + xdy = 0, passes through the intersection of the lines, 2x −-− 3y = 1 and 3x + 2y = 8, then |y(1)| is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 1

  1. Find the point through which the solution curve passes

The curve passes through the intersection of 2x−3y=12x-3y=12x−3y=1 and 3x+2y=8.3x+2y=8.3x+2y=8.

Solve these simultaneously.

From 2x−3y=1 ...(1)2x-3y=1 \,\quad ...(1)2x−3y=1...(1) 3x+2y=8 ...(2)3x+2y=8 \,\quad ...(2)3x+2y=8...(2)

Multiply (1) by 222: 4x−6y=24x-6y=24x−6y=2

Multiply (2) by 333: 9x+6y=249x+6y=249x+6y=24

Add: 13x=26⇒x=213x=26 \Rightarrow x=213x=26⇒x=2

Substitute into (2): 3(2)+2y=8⇒6+2y=8⇒y=13(2)+2y=8 \Rightarrow 6+2y=8 \Rightarrow y=13(2)+2y=8⇒6+2y=8⇒y=1

So the curve passes through (2,1).(2,1).(2,1).


  1. Solve the differential equation

Given: (2xy2−y)dx+xdy=0(2xy^2-y)dx+xdy=0(2xy2−y)dx+xdy=0

Rewrite in differential form: xdydx=−(2xy2−y)=y−2xy2x\frac{dy}{dx}=-(2xy^2-y)=y-2xy^2xdxdy​=−(2xy2−y)=y−2xy2

So, dydx=yx−2y2\frac{dy}{dx}=\frac{y}{x}-2y^2dxdy​=xy​−2y2

This is convenient if we use the substitution v=1y.v=\frac{1}{y}.v=y1​.

Then dvdx=−1y2dydx.\frac{dv}{dx}=-\frac{1}{y^2}\frac{dy}{dx}.dxdv​=−y21​dxdy​.

Using dydx=yx−2y2,\frac{dy}{dx}=\frac{y}{x}-2y^2,dxdy​=xy​−2y2, we get

=−1xy+2.=-\frac{1}{xy}+2.=−xy1​+2.

Since v=1yv=\frac{1}{y}v=y1​, dvdx+1xv=2.\frac{dv}{dx}+\frac{1}{x}v=2.dxdv​+x1​v=2.

This is a linear differential equation.


  1. Solve the linear equation

dvdx+1xv=2\frac{dv}{dx}+\frac{1}{x}v=2dxdv​+x1​v=2

Integrating factor: I.F.=e∫1xdx=eln⁡x=x.\text{I.F.}=e^{\int \frac{1}{x}dx}=e^{\ln x}=x.I.F.=e∫x1​dx=elnx=x.

Multiplying throughout by xxx: xdvdx+v=2xx\frac{dv}{dx}+v=2xxdxdv​+v=2x

Left side is: ddx(xv)=2x\frac{d}{dx}(xv)=2xdxd​(xv)=2x

Integrate: xv=x2+Cxv=x^2+Cxv=x2+C

So, v=x+Cx.v=x+\frac{C}{x}.v=x+xC​.

Since v=1yv=\frac{1}{y}v=y1​, \frac{1}{y}=x+\frac{C}{x}= rac{x^2+C}{x}.

Hence, y=xx2+C.y=\frac{x}{x^2+C}.y=x2+Cx​.


  1. Use the initial condition (2,1)(2,1)(2,1)

Substitute x=2x=2x=2, y=1y=1y=1: 1=24+C1=\frac{2}{4+C}1=4+C2​

So, 4+C=2⇒C=−2.4+C=2 \Rightarrow C=-2.4+C=2⇒C=−2.

Thus the particular solution is y=xx2−2.y=\frac{x}{x^2-2}.y=x2−2x​.


  1. Find y(1)y(1)y(1)

y(1)=11−2=−1.y(1)=\frac{1}{1-2}=-1.y(1)=1−21​=−1.

Therefore, ∣y(1)∣=1.|y(1)|=1.∣y(1)∣=1.


  1. Compare with stored answer

Derived answer: 111

Stored correct answer: 111

They agree.

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