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Differential Equations question

2023 · 1 Feb · Shift 2 · Q39
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  5. /2023 · 1 Feb · Shift 2 · Q39

Differential Equations question

2023 · 1 Feb · Shift 2 · Q39

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let αx=exp⁡(xβyγ)\alpha x=\exp \left(x^{\beta} y^{\gamma}\right)αx=exp(xβyγ) be the solution of the differential equation 2x2y dy−(1−xy2)dx=0,x>0,y(2)=log⁡e22 x^{2} y \mathrm{~d} y-\left(1-x y^{2}\right) \mathrm{d} x=0, x \gt 0,y(2)=\sqrt{\log _{e} 2}2x2y dy−(1−xy2)dx=0,x>0,y(2)=loge​2​. Then α+β−γ\alpha+\beta-\gammaα+β−γ equals :
  1. A
    1
  2. B
    0
  3. C
    3
  4. D
    −1-1−1
View written solutionFree

Correct answer: A

  1. Rewrite the differential equation

Given 2x2y dy−(1−xy2) dx=0.2x^2y\,dy-(1-xy^2)\,dx=0.2x2ydy−(1−xy2)dx=0.

Rearrange in derivative form: 2x2ydydx−(1−xy2)=02x^2y\frac{dy}{dx}-(1-xy^2)=02x2ydxdy​−(1−xy2)=0 2x2ydydx=1−xy2.2x^2y\frac{dy}{dx}=1-xy^2.2x2ydxdy​=1−xy2.

So, dydx=1−xy22x2y.\frac{dy}{dx}=\frac{1-xy^2}{2x^2y}.dxdy​=2x2y1−xy2​.

  1. Make a substitution

Let u=y2.u=y^2.u=y2. Then dudx=2ydydx.\frac{du}{dx}=2y\frac{dy}{dx}.dxdu​=2ydxdy​.

Using the differential equation, 2ydydx=1−xy2x2=1−xux2.2y\frac{dy}{dx}=\frac{1-xy^2}{x^2}=\frac{1-xu}{x^2}.2ydxdy​=x21−xy2​=x21−xu​.

Hence, dudx=1x2−ux.\frac{du}{dx}=\frac{1}{x^2}-\frac{u}{x}.dxdu​=x21​−xu​.

So we get the linear differential equation dudx+1xu=1x2.\frac{du}{dx}+\frac{1}{x}u=\frac{1}{x^2}.dxdu​+x1​u=x21​.

  1. Solve the linear equation

Integrating factor: I.F.=e∫1x dx=x(x>0).\text{I.F.}=e^{\int \frac1x\,dx}=x \qquad (x>0).I.F.=e∫x1​dx=x(x>0).

Multiplying throughout by xxx: xdudx+u=1x.x\frac{du}{dx}+u=\frac{1}{x}.xdxdu​+u=x1​.

Thus, ddx(xu)=1x.\frac{d}{dx}(xu)=\frac{1}{x}.dxd​(xu)=x1​.

Integrating, xu=ln⁡x+C.xu=\ln x + C.xu=lnx+C.

Therefore, u=ln⁡x+Cx.u=\frac{\ln x + C}{x}.u=xlnx+C​.

Since u=y2u=y^2u=y2, y2=ln⁡x+Cx.y^2=\frac{\ln x + C}{x}.y2=xlnx+C​.

  1. Use the initial condition

Given y(2)=ln⁡2.y(2)=\sqrt{\ln 2}.y(2)=ln2​.

So, y2(2)=ln⁡2.y^2(2)=\ln 2.y2(2)=ln2.

Substitute in the general solution: ln⁡2=ln⁡2+C2.\ln 2=\frac{\ln 2 + C}{2}.ln2=2ln2+C​.

Therefore, 2ln⁡2=ln⁡2+C2\ln 2=\ln 2 + C2ln2=ln2+C C=ln⁡2.C=\ln 2.C=ln2.

Hence, y2=ln⁡x+ln⁡2x=ln⁡(2x)x.y^2=\frac{\ln x + \ln 2}{x}=\frac{\ln(2x)}{x}.y2=xlnx+ln2​=xln(2x)​.

So, xy2=ln⁡(2x).xy^2=\ln(2x).xy2=ln(2x).

  1. Convert to the required form

Exponentiating both sides, exy2=2x.e^{xy^2}=2x.exy2=2x.

This matches αx=exp⁡(xβyγ).\alpha x=\exp\left(x^{\beta}y^{\gamma}\right).αx=exp(xβyγ).

Comparing, α=2,β=1,γ=2.\alpha=2,\quad \beta=1,\quad \gamma=2.α=2,β=1,γ=2.

Thus, α+β−γ=2+1−2=1.\alpha+\beta-\gamma=2+1-2=1.α+β−γ=2+1−2=1.

  1. Check options

The value is 111, which corresponds to Option A.

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