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Differential Equations question

2022 · 26 Jun · Shift 2 · Q33
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  5. /2022 · 26 Jun · Shift 2 · Q33

Differential Equations question

2022 · 26 Jun · Shift 2 · Q33

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
If the solution of the differential equation dydx+ex(x2−2)y=(x2−2x)(x2−2)e2x{{dy} \over {dx}} + {e^x}\left( {{x^2} - 2} \right)y = \left( {{x^2} - 2x} \right)\left( {{x^2} - 2} \right){e^{2x}}dxdy​+ex(x2−2)y=(x2−2x)(x2−2)e2x satisfies y(0)=0y(0) = 0y(0)=0, then the value of y(2) is ‾\underline{\hspace{2cm}}​.
  1. A
    −-− 1
  2. B
    1
  3. C
    0
  4. D
    e
View written solutionFree

Correct answer: C

  1. Given differential equation
dydx+ex(x2−2)y=(x2−2x)(x2−2)e2x\frac{dy}{dx}+e^x(x^2-2)y=(x^2-2x)(x^2-2)e^{2x}dxdy​+ex(x2−2)y=(x2−2x)(x2−2)e2x

with initial condition

y(0)=0.y(0)=0.y(0)=0.

This is a linear differential equation of the form

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

where

P(x)=ex(x2−2),Q(x)=(x2−2x)(x2−2)e2x.P(x)=e^x(x^2-2), \qquad Q(x)=(x^2-2x)(x^2-2)e^{2x}.P(x)=ex(x2−2),Q(x)=(x2−2x)(x2−2)e2x.
  1. Look for a useful substitution

Notice that the right-hand side contains e2x(x2−2)e^{2x}(x^2-2)e2x(x2−2), and the coefficient of yyy contains ex(x2−2)e^x(x^2-2)ex(x2−2). This suggests trying

y=vex.y=v e^x.y=vex.

Then

dydx=exdvdx+vex.\frac{dy}{dx}=e^x\frac{dv}{dx}+ve^x.dxdy​=exdxdv​+vex.

Substitute into the differential equation:

exdvdx+vex+ex(x2−2)(vex)=(x2−2x)(x2−2)e2x.e^x\frac{dv}{dx}+ve^x+e^x(x^2-2)(ve^x)=(x^2-2x)(x^2-2)e^{2x}.exdxdv​+vex+ex(x2−2)(vex)=(x2−2x)(x2−2)e2x.

Divide throughout by exe^xex:

dvdx+v+v(x2−2)ex=(x2−2x)(x2−2)ex.\frac{dv}{dx}+v+v(x^2-2)e^x=(x^2-2x)(x^2-2)e^x.dxdv​+v+v(x2−2)ex=(x2−2x)(x2−2)ex.

This does not simplify nicely. So instead, we solve by integrating factor.


  1. Find the integrating factor

For

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

integrating factor is

I.F.=e∫P(x)dx=e∫ex(x2−2)dx.\text{I.F.}=e^{\int P(x)dx}=e^{\int e^x(x^2-2)dx}.I.F.=e∫P(x)dx=e∫ex(x2−2)dx.

Now compute

∫ex(x2−2)dx.\int e^x(x^2-2)dx.∫ex(x2−2)dx.

We use the known result:

ddx[ex(x2−2x)]=ex(x2−2x)+ex(2x−2)=ex(x2−2).\frac{d}{dx}\big[e^x(x^2-2x)\big]=e^x(x^2-2x)+e^x(2x-2)=e^x(x^2-2).dxd​[ex(x2−2x)]=ex(x2−2x)+ex(2x−2)=ex(x2−2).

Hence,

∫ex(x2−2)dx=ex(x2−2x).\int e^x(x^2-2)dx=e^x(x^2-2x).∫ex(x2−2)dx=ex(x2−2x).

So the integrating factor is

I.F.=eex(x2−2x).\text{I.F.}=e^{e^x(x^2-2x)}.I.F.=eex(x2−2x).
  1. Write the solution form

Multiplying the differential equation by the integrating factor,

ddx[y eex(x2−2x)]=Q(x) eex(x2−2x).\frac{d}{dx}\left[y\,e^{e^x(x^2-2x)}\right]=Q(x)\,e^{e^x(x^2-2x)}.dxd​[yeex(x2−2x)]=Q(x)eex(x2−2x).

That is,

ddx[y eex(x2−2x)]=(x2−2x)(x2−2)e2x eex(x2−2x).\frac{d}{dx}\left[y\,e^{e^x(x^2-2x)}\right]=(x^2-2x)(x^2-2)e^{2x}\,e^{e^x(x^2-2x)}.dxd​[yeex(x2−2x)]=(x2−2x)(x2−2)e2xeex(x2−2x).

Now observe:

Let

u=ex(x2−2x).u=e^x(x^2-2x).u=ex(x2−2x).

Then

dνdx=ex(x2−2).\frac{d\nu}{dx}=e^x(x^2-2).dxdν​=ex(x2−2).

So the right-hand side becomes

(x2−2x)ex⋅eν⋅dνdx.(x^2-2x)e^x\cdot e^{\nu}\cdot \frac{d\nu}{dx}.(x2−2x)ex⋅eν⋅dxdν​.

But since

ν=ex(x2−2x),\nu=e^x(x^2-2x),ν=ex(x2−2x),

we have

(x2−2x)ex=ν.(x^2-2x)e^x=\nu.(x2−2x)ex=ν.

Therefore,

ddx[yeν]=νeνdνdx.\frac{d}{dx}\left[y e^{\nu}\right]=\nu e^{\nu}\frac{d\nu}{dx}.dxd​[yeν]=νeνdxdν​.

Hence,

yeν=∫νeν dν.ye^{\nu}=\int \nu e^{\nu}\,d\nu.yeν=∫νeνdν.

Now,

∫νeνdν=eν(ν−1)+C.\int \nu e^{\nu}d\nu=e^{\nu}(\nu-1)+C.∫νeνdν=eν(ν−1)+C.

Thus,

yeν=eν(ν−1)+Cye^{\nu}=e^{\nu}(\nu-1)+Cyeν=eν(ν−1)+C

or

y=ν−1+Ce−ν.y=\nu-1+Ce^{-\nu}.y=ν−1+Ce−ν.

Substituting back,

y=ex(x2−2x)−1+Ce−ex(x2−2x).y=e^x(x^2-2x)-1+Ce^{-e^x(x^2-2x)}.y=ex(x2−2x)−1+Ce−ex(x2−2x).
  1. Use the initial condition y(0)=0y(0)=0y(0)=0

At x=0x=0x=0,

e0(02−2⋅0)=0.e^0(0^2-2\cdot 0)=0.e0(02−2⋅0)=0.

So

y(0)=0−1+Ce0=−1+C.y(0)=0-1+Ce^0=-1+C.y(0)=0−1+Ce0=−1+C.

Given y(0)=0y(0)=0y(0)=0,

−1+C=0  ⟹  C=1.-1+C=0 \implies C=1.−1+C=0⟹C=1.

Hence,

y=ex(x2−2x)−1+e−ex(x2−2x).y=e^x(x^2-2x)-1+e^{-e^x(x^2-2x)}.y=ex(x2−2x)−1+e−ex(x2−2x).
  1. Find y(2)y(2)y(2)

At x=2x=2x=2,

x2−2x=4−4=0.x^2-2x=4-4=0.x2−2x=4−4=0.

Therefore,

ex(x2−2x)=e2⋅0=0.e^x(x^2-2x)=e^2\cdot 0=0.ex(x2−2x)=e2⋅0=0.

So

y(2)=0−1+e0=−1+1=0.y(2)=0-1+e^0=-1+1=0.y(2)=0−1+e0=−1+1=0.
  1. Compare with options
y(2)=0y(2)=0y(2)=0

So the correct option is:

C: 000


  1. Comparison with stored correct answer

Stored correct answer: C

Our derived answer: C

They match.

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