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Differential Equations question

2022 · 24 Jun · Shift 1 · Q32
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  5. /2022 · 24 Jun · Shift 1 · Q32

Differential Equations question

2022 · 24 Jun · Shift 1 · Q32

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
If x = x(y) is the solution of the differential equation ydxdy=2x+y3(y+1)ey, x(1)=0y{{dx} \over {dy}} = 2x + {y^3}(y + 1){e^y},\,x(1) = 0ydydx​=2x+y3(y+1)ey,x(1)=0; then x(e) is equal to :
  1. A
    e3(ee−1){e^3}({e^e} - 1)e3(ee−1)
  2. B
    ee(e3−1){e^e}({e^3} - 1)ee(e3−1)
  3. C
    e2(ee+1){e^2}({e^e} + 1)e2(ee+1)
  4. D
    ee(e2−1){e^e}({e^2} - 1)ee(e2−1)
View written solutionFree

Correct answer: A

  1. Given differential equation

We have ydxdy=2x+y3(y+1)ey,y\frac{dx}{dy}=2x+y^3(y+1)e^y,ydydx​=2x+y3(y+1)ey, with initial condition x(1)=0.x(1)=0.x(1)=0.

We need to find x(e)x(e)x(e).


  1. Rewrite in standard linear form

Divide both sides by yyy (assuming y≠0y\neq 0y=0, and here we work near y=1y=1y=1 to y=ey=ey=e): dxdy−2yx=y2(y+1)ey.\frac{dx}{dy}-\frac{2}{y}x=y^2(y+1)e^y.dydx​−y2​x=y2(y+1)ey.

So this is a linear differential equation in xxx as a function of yyy: dxdy+P(y)x=Q(y),\frac{dx}{dy}+P(y)x=Q(y),dydx​+P(y)x=Q(y), where P(y)=−2y,Q(y)=y2(y+1)ey.P(y)=-\frac{2}{y}, \qquad Q(y)=y^2(y+1)e^y.P(y)=−y2​,Q(y)=y2(y+1)ey.


  1. Find the integrating factor

The integrating factor is I.F.=e∫P(y)dy=e∫−2ydy=e−2ln⁡y=y−2.\mathrm{I.F.}=e^{\int P(y)dy}=e^{\int -\frac{2}{y}dy}=e^{-2\ln y}=y^{-2}.I.F.=e∫P(y)dy=e∫−y2​dy=e−2lny=y−2.


  1. Multiply the equation by the integrating factor

Multiplying throughout by y−2y^{-2}y−2: y−2dxdy−2yy−2x=(y+1)ey.y^{-2}\frac{dx}{dy}-\frac{2}{y}y^{-2}x=(y+1)e^y.y−2dydx​−y2​y−2x=(y+1)ey.

The left-hand side becomes ddy(xy−2).\frac{d}{dy}(x y^{-2}).dyd​(xy−2). Hence, ddy(xy−2)=(y+1)ey.\frac{d}{dy}(x y^{-2})=(y+1)e^y.dyd​(xy−2)=(y+1)ey.


  1. Integrate both sides

Integrate with respect to yyy: xy−2=∫(y+1)ey dy+C.x y^{-2}=\int (y+1)e^y\,dy + C.xy−2=∫(y+1)eydy+C.

Now, ∫(y+1)ey dy=yey,\int (y+1)e^y\,dy = ye^y,∫(y+1)eydy=yey, because ddy(yey)=ey+yey=(y+1)ey.\frac{d}{dy}(ye^y)=e^y+ye^y=(y+1)e^y.dyd​(yey)=ey+yey=(y+1)ey.

Therefore, xy−2=yey+C.x y^{-2}=ye^y+C.xy−2=yey+C. So, x=y2(yey+C).x=y^2(ye^y+C).x=y2(yey+C).


  1. Use the initial condition

Given x(1)=0x(1)=0x(1)=0: 0=12(1⋅e1+C)=e+C.0=1^2(1\cdot e^1+C)=e+C.0=12(1⋅e1+C)=e+C. So, C=−e.C=-e.C=−e.

Thus, x=y2(yey−e).x=y^2(ye^y-e).x=y2(yey−e).


  1. Evaluate at y=ey=ey=e

x(e)=e2(e⋅ee−e)=e2⋅e (ee−1)=e3(ee−1).x(e)=e^2\big(e\cdot e^e-e\big)=e^2\cdot e\,(e^e-1)=e^3(e^e-1).x(e)=e2(e⋅ee−e)=e2⋅e(ee−1)=e3(ee−1).


  1. Compare with the options

We obtained x(e)=e3(ee−1).x(e)=e^3(e^e-1).x(e)=e3(ee−1). This matches:

Option A: e3(ee−1).e^3(e^e-1).e3(ee−1).

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