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Differential Equations question

2020 · 7 Jan · Shift 1 · Q35
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  5. /2020 · 7 Jan · Shift 1 · Q35

Differential Equations question

2020 · 7 Jan · Shift 1 · Q35

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
If y = y(x) is the solution of the differential equation, ey(dydx−1)=ex{e^y}\left( {{{dy} \over {dx}} - 1} \right) = {e^x}ey(dxdy​−1)=ex such that y(0) = 0, then y(1) is equal to:
  1. A
    2 + loge2
  2. B
    loge2
  3. C
    1 + loge2
  4. D
    2e
View written solutionFree

Correct answer: C

  1. Given differential equation

ey(dydx−1)=exe^y\left(\frac{dy}{dx}-1\right)=e^xey(dxdy​−1)=ex

We need to solve it with the initial condition

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

and then find y(1)y(1)y(1).


  1. Rewrite the equation

Expand the left side:

eydydx−ey=exe^y\frac{dy}{dx}-e^y=e^xeydxdy​−ey=ex

Now observe that

ddx(ey)=eydydx\frac{d}{dx}(e^y)=e^y\frac{dy}{dx}dxd​(ey)=eydxdy​

So the differential equation becomes

ddx(ey)−ey=ex\frac{d}{dx}(e^y)-e^y=e^xdxd​(ey)−ey=ex

Let

u=eyu=e^yu=ey

Then the equation reduces to a linear differential equation:

dudx−u=ex\frac{du}{dx}-u=e^xdxdu​−u=ex


  1. Solve the linear differential equation

We have

dudx−u=ex\frac{du}{dx}-u=e^xdxdu​−u=ex

The integrating factor is

IF=e∫−1 dx=e−xIF=e^{\int -1\,dx}=e^{-x}IF=e∫−1dx=e−x

Multiply both sides by e−xe^{-x}e−x:

e−xdudx−e−xu=1e^{-x}\frac{du}{dx}-e^{-x}u=1e−xdxdu​−e−xu=1

The left side is

ddx(ue−x)=1\frac{d}{dx}(ue^{-x})=1dxd​(ue−x)=1

Integrating both sides:

ue−x=x+Cue^{-x}=x+Cue−x=x+C

So,

u=ex(x+C)u=e^x(x+C)u=ex(x+C)

Since u=eyu=e^yu=ey,

ey=ex(x+C)e^y=e^x(x+C)ey=ex(x+C)

Taking natural logarithm,

y=x+ln⁡(x+C)y=x+\ln(x+C)y=x+ln(x+C)


  1. Use the initial condition

Given y(0)=0y(0)=0y(0)=0:

0=0+ln⁡C0=0+\ln C0=0+lnC

So,

ln⁡C=0⇒C=1\ln C=0 \Rightarrow C=1lnC=0⇒C=1

Hence the solution is

y=x+ln⁡(x+1)y=x+\ln(x+1)y=x+ln(x+1)


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

Substitute x=1x=1x=1:

y(1)=1+ln⁡(2)y(1)=1+\ln(2)y(1)=1+ln(2)


  1. Compare with options
  • A: 2+ln⁡22+\ln 22+ln2
  • B: ln⁡2\ln 2ln2
  • C: 1+ln⁡21+\ln 21+ln2
  • D: 2e2e2e

Thus the correct option is:

C\boxed{\text{C}}C​

and

y(1)=1+ln⁡2\boxed{y(1)=1+\ln 2}y(1)=1+ln2​

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