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Differential Equations question

2024 · 1 Feb · Shift 2 · Q54
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  5. /2024 · 1 Feb · Shift 2 · Q54

Differential Equations question

2024 · 1 Feb · Shift 2 · Q54

JEE MainMathematicsDifferential EquationsNumerical+4 / −1
If dx dy=1+x−y2y,x(1)=1\frac{\mathrm{d} x}{\mathrm{~d} y}=\frac{1+x-y^2}{y}, x(1)=1 dydx​=y1+x−y2​,x(1)=1, then 5x(2)5 x(2)5x(2) is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 5

  1. Given differential equation

We have

dxdy=1+x−y2y,x(1)=1.\frac{dx}{dy}=\frac{1+x-y^2}{y}, \qquad x(1)=1.dydx​=y1+x−y2​,x(1)=1.

We need to find x(2)x(2)x(2) and then compute 5x(2)5x(2)5x(2).


  1. Rewrite in linear form

Write the equation as

dxdy=1+x−y2y=1y+xy−y.\frac{dx}{dy}=\frac{1+x-y^2}{y}=\frac{1}{y}+\frac{x}{y}-y.dydx​=y1+x−y2​=y1​+yx​−y.

So,

dxdy−1yx=1y−y.\frac{dx}{dy}-\frac{1}{y}x=\frac{1}{y}-y.dydx​−y1​x=y1​−y.

This is a linear differential equation in xxx as a function of yyy.


  1. Find the integrating factor

For

dxdy+P(y)x=Q(y),\frac{dx}{dy}+P(y)x=Q(y),dydx​+P(y)x=Q(y),

we have

P(y)=−1y.P(y)=-\frac{1}{y}.P(y)=−y1​.

Hence the integrating factor is

I.F.=e∫−1y dy=e−ln⁡y=1y\text{I.F.}=e^{\int -\frac{1}{y}\,dy}=e^{-\ln y}=\frac{1}{y}I.F.=e∫−y1​dy=e−lny=y1​

(taking y>0y>0y>0 since the condition is given at y=1y=1y=1).


  1. Multiply throughout by the integrating factor

Multiplying the equation

dxdy−xy=1y−y\frac{dx}{dy}-\frac{x}{y}=\frac{1}{y}-ydydx​−yx​=y1​−y

by 1y\frac{1}{y}y1​, we get

1ydxdy−xy2=1y2−1.\frac{1}{y}\frac{dx}{dy}-\frac{x}{y^2}=\frac{1}{y^2}-1.y1​dydx​−y2x​=y21​−1.

Notice that

ddy(xy)=1ydxdy−xy2.\frac{d}{dy}\left(\frac{x}{y}\right)=\frac{1}{y}\frac{dx}{dy}-\frac{x}{y^2}.dyd​(yx​)=y1​dydx​−y2x​.

Therefore,

ddy(xy)=1y2−1.\frac{d}{dy}\left(\frac{x}{y}\right)=\frac{1}{y^2}-1.dyd​(yx​)=y21​−1.
  1. Integrate

Integrating both sides,

xy=∫(1y2−1)dy=∫y−2dy−∫1 dy.\frac{x}{y}=\int \left(\frac{1}{y^2}-1\right)dy =\int y^{-2}dy-\int 1\,dy.yx​=∫(y21​−1)dy=∫y−2dy−∫1dy.

Thus,

xy=−1y−y+C.\frac{x}{y}=-\frac{1}{y}-y+C.yx​=−y1​−y+C.

Multiply by yyy:

x=−1−y2+Cy.x=-1-y^2+Cy.x=−1−y2+Cy.
  1. Use the initial condition

Given x(1)=1x(1)=1x(1)=1,

1=−1−1+C(1).1=-1-1+C(1).1=−1−1+C(1).

So,

1=−2+C  ⟹  C=3.1=-2+C \implies C=3.1=−2+C⟹C=3.

Hence,

x(y)=−1−y2+3y.x(y)=-1-y^2+3y.x(y)=−1−y2+3y.
  1. Find x(2)x(2)x(2)
x(2)=−1−4+6=1.x(2)=-1-4+6=1.x(2)=−1−4+6=1.

Therefore,

5x(2)=5⋅1=5.5x(2)=5\cdot 1=5.5x(2)=5⋅1=5.
  1. Comparison with stored answer

Our derived answer is 555, which matches the stored correct answer.

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