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Differential Equations question

2022 · 28 Jun · Shift 1 · Q31
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  5. /2022 · 28 Jun · Shift 1 · Q31

Differential Equations question

2022 · 28 Jun · Shift 1 · Q31

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let the solution curve y=y(x)y = y(x)y=y(x) of the differential equation [xx2−y2+eyx]xdydx=x+[xx2−y2+eyx]y\left[ {{x \over {\sqrt {{x^2} - {y^2}} }} + {e^{{y \over x}}}} \right]x{{dy} \over {dx}} = x + \left[ {{x \over {\sqrt {{x^2} - {y^2}} }} + {e^{{y \over x}}}} \right]y[x2−y2​x​+exy​]xdxdy​=x+[x2−y2​x​+exy​]y pass through the points (1, 0) and (2 α\alphaα, α\alphaα), α\alphaα> 0. Then α\alphaα is equal to
  1. A
    12exp⁡(π6+e−1){1 \over 2}\exp \left( {{\pi \over 6} + \sqrt e - 1} \right)21​exp(6π​+e​−1)
  2. B
    12exp⁡(π6+e−1){1 \over 2}\exp \left( {{\pi \over 6} + e - 1} \right)21​exp(6π​+e−1)
  3. C
    exp⁡(π6+e+1)\exp \left( {{\pi \over 6} + \sqrt e + 1} \right)exp(6π​+e​+1)
  4. D
    2exp⁡(π3+e−1)2\exp \left( {{\pi \over 3} + \sqrt e - 1} \right)2exp(3π​+e​−1)
View written solutionFree

Correct answer: A

  1. Rewrite the differential equation

Given [xx2−y2+ey/x]xdydx=x+[xx2−y2+ey/x]y.\left[ \frac{x}{\sqrt{x^2-y^2}}+e^{y/x}\right]x\frac{dy}{dx}=x+\left[ \frac{x}{\sqrt{x^2-y^2}}+e^{y/x}\right]y.[x2−y2​x​+ey/x]xdxdy​=x+[x2−y2​x​+ey/x]y.

Let A=xx2−y2+ey/x.A=\frac{x}{\sqrt{x^2-y^2}}+e^{y/x}.A=x2−y2​x​+ey/x. Then the equation becomes A xdydx=x+Ay.A\,x\frac{dy}{dx}=x+A y.Axdxdy​=x+Ay. So, A(xdydx−y)=x.A\left(x\frac{dy}{dx}-y\right)=x.A(xdxdy​−y)=x.

Hence, xdydx−y=xA.x\frac{dy}{dx}-y=\frac{x}{A}.xdxdy​−y=Ax​.

  1. Use the substitution v=yx\displaystyle v=\frac{y}{x}v=xy​

Let y=vx⇒dydx=v+xdvdx.y=vx \quad \Rightarrow \quad \frac{dy}{dx}=v+x\frac{dv}{dx}.y=vx⇒dxdy​=v+xdxdv​. Thus, xdydx−y=x(v+xdvdx)−vx=x2dvdx.x\frac{dy}{dx}-y=x\left(v+x\frac{dv}{dx}\right)-vx=x^2\frac{dv}{dx}.xdxdy​−y=x(v+xdxdv​)−vx=x2dxdv​.

Now simplify AAA: x2−y2=x2(1−v2)=x1−v2\sqrt{x^2-y^2}=\sqrt{x^2(1-v^2)}=x\sqrt{1-v^2}x2−y2​=x2(1−v2)​=x1−v2​ (since the given points have positive xxx, we take this directly on the relevant curve segment). Therefore, xx2−y2=11−v2.\frac{x}{\sqrt{x^2-y^2}}=\frac{1}{\sqrt{1-v^2}}.x2−y2​x​=1−v2​1​. So, A=11−v2+ev.A=\frac{1}{\sqrt{1-v^2}}+e^v.A=1−v2​1​+ev.

Hence the DE becomes x2dvdx=x11−v2+ev.x^2\frac{dv}{dx}=\frac{x}{\frac{1}{\sqrt{1-v^2}}+e^v}.x2dxdv​=1−v2​1​+evx​. So, xdvdx=111−v2+ev.x\frac{dv}{dx}=\frac{1}{\frac{1}{\sqrt{1-v^2}}+e^v}.xdxdv​=1−v2​1​+ev1​.

Equivalently, dxx=(11−v2+ev)dv.\frac{dx}{x}=\left(\frac{1}{\sqrt{1-v^2}}+e^v\right)dv.xdx​=(1−v2​1​+ev)dv.

  1. Integrate

Integrating both sides, ∫dxx=∫(11−v2+ev)dv.\int \frac{dx}{x}=\int \left(\frac{1}{\sqrt{1-v^2}}+e^v\right)dv.∫xdx​=∫(1−v2​1​+ev)dv. Therefore, ln⁡x=sin⁡−1v+ev+C.\ln x=\sin^{-1}v+e^v+C.lnx=sin−1v+ev+C. Since v=yxv=\dfrac{y}{x}v=xy​, ln⁡x=sin⁡−1(yx)+ey/x+C.\ln x=\sin^{-1}\left(\frac{y}{x}\right)+e^{y/x}+C.lnx=sin−1(xy​)+ey/x+C.

  1. Use the point (1,0)(1,0)(1,0)

At (x,y)=(1,0)(x,y)=(1,0)(x,y)=(1,0), ln⁡1=sin⁡−1(0)+e0+C\ln 1=\sin^{-1}(0)+e^0+Cln1=sin−1(0)+e0+C 0=0+1+C0=0+1+C0=0+1+C So, C=−1.C=-1.C=−1.

Thus the solution is ln⁡x=sin⁡−1(yx)+ey/x−1.\ln x=\sin^{-1}\left(\frac{y}{x}\right)+e^{y/x}-1.lnx=sin−1(xy​)+ey/x−1.

  1. Use the point (2α,α)(2\alpha,\alpha)(2α,α)

At (x,y)=(2α,α)(x,y)=(2\alpha,\alpha)(x,y)=(2α,α), yx=α2α=12.\frac{y}{x}=\frac{\alpha}{2\alpha}=\frac12.xy​=2αα​=21​. So, ln⁡(2α)=sin⁡−1(12)+e1/2−1.\ln(2\alpha)=\sin^{-1}\left(\frac12\right)+e^{1/2}-1.ln(2α)=sin−1(21​)+e1/2−1. Now, sin⁡−1(12)=π6,e1/2=e.\sin^{-1}\left(\frac12\right)=\frac{\pi}{6}, \qquad e^{1/2}=\sqrt e.sin−1(21​)=6π​,e1/2=e​. Hence, ln⁡(2α)=π6+e−1.\ln(2\alpha)=\frac{\pi}{6}+\sqrt e-1.ln(2α)=6π​+e​−1. Exponentiating, 2α=exp⁡(π6+e−1).2\alpha=\exp\left(\frac{\pi}{6}+\sqrt e-1\right).2α=exp(6π​+e​−1). Therefore, α=12exp⁡(π6+e−1).\alpha=\frac12\exp\left(\frac{\pi}{6}+\sqrt e-1\right).α=21​exp(6π​+e​−1).

  1. Match with the options

This is exactly Option A.

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