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Differential Equations question

2021 · 26 Aug · Shift 2 · Q27
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  5. /2021 · 26 Aug · Shift 2 · Q27

Differential Equations question

2021 · 26 Aug · Shift 2 · Q27

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let y(x) be the solution of the differential equation 2x2 dy + (ey −-− 2x)dx = 0, x > 0. If y(e) = 1, then y(1) is equal to :
  1. A
    0
  2. B
    2
  3. C
    loge 2
  4. D
    loge (2e)
View written solutionFree

Correct answer: C

  1. Rewrite the differential equation

Given: 2x2 dy+(ey−2x) dx=0,x>02x^2\,dy+(e^y-2x)\,dx=0, \qquad x>02x2dy+(ey−2x)dx=0,x>0

Divide by dxdxdx: 2x2dydx+ey−2x=02x^2\frac{dy}{dx}+e^y-2x=02x2dxdy​+ey−2x=0

So, 2x2dydx=2x−ey2x^2\frac{dy}{dx}=2x-e^y2x2dxdy​=2x−ey dydx=1x−ey2x2\frac{dy}{dx}=\frac{1}{x}-\frac{e^y}{2x^2}dxdy​=x1​−2x2ey​

  1. Use a substitution

Let u=e−yu=e^{-y}u=e−y Then dudx=−e−ydydx=−udydx\frac{du}{dx}=-e^{-y}\frac{dy}{dx}=-u\frac{dy}{dx}dxdu​=−e−ydxdy​=−udxdy​

From dydx=1x−ey2x2\frac{dy}{dx}=\frac{1}{x}-\frac{e^y}{2x^2}dxdy​=x1​−2x2ey​ we get dudx=−u(1x−ey2x2)\frac{du}{dx}=-u\left(\frac{1}{x}-\frac{e^y}{2x^2}\right)dxdu​=−u(x1​−2x2ey​) Since u=e−yu=e^{-y}u=e−y, we have uey=1ue^y=1uey=1. Hence dudx=−ux+12x2\frac{du}{dx}=-\frac{u}{x}+\frac{1}{2x^2}dxdu​=−xu​+2x21​

Therefore the linear differential equation in uuu is: dudx+1xu=12x2\frac{du}{dx}+\frac{1}{x}u=\frac{1}{2x^2}dxdu​+x1​u=2x21​

  1. Solve the linear equation

Integrating factor: I.F.=e∫1xdx=eln⁡x=x\text{I.F.}=e^{\int \frac{1}{x}dx}=e^{\ln x}=xI.F.=e∫x1​dx=elnx=x (since x>0x>0x>0)

Multiply throughout by xxx: xdudx+u=12xx\frac{du}{dx}+u=\frac{1}{2x}xdxdu​+u=2x1​ So, ddx(xu)=12x\frac{d}{dx}(xu)=\frac{1}{2x}dxd​(xu)=2x1​

Integrate: xu=12ln⁡x+Cxu=\frac{1}{2}\ln x+Cxu=21​lnx+C

Substitute back u=e−yu=e^{-y}u=e−y: xe−y=12ln⁡x+Cxe^{-y}=\frac{1}{2}\ln x+Cxe−y=21​lnx+C

  1. Use the condition y(e)=1y(e)=1y(e)=1

At x=ex=ex=e, y=1y=1y=1: e⋅e−1=12ln⁡e+Ce\cdot e^{-1}=\frac{1}{2}\ln e+Ce⋅e−1=21​lne+C 1=12+C1=\frac{1}{2}+C1=21​+C C=12C=\frac{1}{2}C=21​

Hence, xe−y=12(ln⁡x+1)xe^{-y}=\frac{1}{2}(\ln x+1)xe−y=21​(lnx+1)

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

At x=1x=1x=1: 1⋅e−y(1)=12(ln⁡1+1)=121\cdot e^{-y(1)}=\frac{1}{2}(\ln 1+1)=\frac{1}{2}1⋅e−y(1)=21​(ln1+1)=21​ So, e−y(1)=12e^{-y(1)}=\frac{1}{2}e−y(1)=21​ −y(1)=ln⁡(12)=−ln⁡2-y(1)=\ln\left(\frac{1}{2}\right)=-\ln 2−y(1)=ln(21​)=−ln2 y(1)=ln⁡2y(1)=\ln 2y(1)=ln2

  1. Match with options

y(1)=ln⁡2=log⁡e2y(1)=\ln 2=\log_e 2y(1)=ln2=loge​2 So the correct option is C.

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