Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Differential Equations question

2020 · 3 Sep · Shift 1 · Q22
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Differential Equations
  5. /2020 · 3 Sep · Shift 1 · Q22

Differential Equations question

2020 · 3 Sep · Shift 1 · Q22

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
The solution curve of the differential equation, (1 + e-x)(1 + y2) dydx{{dy} \over {dx}}dxdy​ = y2, which passes through the point (0, 1), is :
  1. A
    y2 + 1 = y (log⁡e(1+e−x2)+2)\left( {{{\log }_e}\left( {{{1 + {e^{ - x}}} \over 2}} \right) + 2} \right)(loge​(21+e−x​)+2)
  2. B
    y2 + 1 = y (log⁡e(1+ex2)+2)\left( {{{\log }_e}\left( {{{1 + {e^{ x}}} \over 2}} \right) + 2} \right)(loge​(21+ex​)+2)
  3. C
    y2 = 1 + ylog⁡e(1+e−x2){y{{\log }_e}\left( {{{1 + {e^{ - x}}} \over 2}} \right)}yloge​(21+e−x​)
  4. D
    y2 = 1 + ylog⁡e(1+ex2){y{{\log }_e}\left( {{{1 + {e^{ x}}} \over 2}} \right)}yloge​(21+ex​)
View written solutionFree

Correct answer: D

  1. Given differential equation

    (1+e−x)(1+y2)dydx=y2(1+e^{-x})(1+y^2)\frac{dy}{dx}=y^2(1+e−x)(1+y2)dxdy​=y2

    We need the solution passing through (0,1)(0,1)(0,1).

  2. Separate variables

    Rearranging,

    1+y2y2 dy=dx1+e−x\frac{1+y^2}{y^2}\,dy = \frac{dx}{1+e^{-x}}y21+y2​dy=1+e−xdx​

    Since

    1+y2y2=1+1y2,\frac{1+y^2}{y^2}=1+\frac{1}{y^2},y21+y2​=1+y21​,

    we get

    (1+1y2)dy=dx1+e−x\left(1+\frac{1}{y^2}\right)dy = \frac{dx}{1+e^{-x}}(1+y21​)dy=1+e−xdx​

  3. Integrate both sides

    Left side:

    ∫(1+1y2)dy=∫dy+∫y−2dy=y−1y\int \left(1+\frac{1}{y^2}\right)dy = \int dy + \int y^{-2}dy = y-\frac{1}{y}∫(1+y21​)dy=∫dy+∫y−2dy=y−y1​

    Right side:

    ∫dx1+e−x\int \frac{dx}{1+e^{-x}}∫1+e−xdx​

    Multiply numerator and denominator by exe^xex:

    11+e−x=ex1+ex\frac{1}{1+e^{-x}}=\frac{e^x}{1+e^x}1+e−x1​=1+exex​

    So,

    ∫dx1+e−x=∫ex1+exdx=ln⁡(1+ex)+C\int \frac{dx}{1+e^{-x}} = \int \frac{e^x}{1+e^x}dx = \ln(1+e^x)+C∫1+e−xdx​=∫1+exex​dx=ln(1+ex)+C

    Hence the general solution is

    y−1y=ln⁡(1+ex)+Cy-\frac{1}{y}=\ln(1+e^x)+Cy−y1​=ln(1+ex)+C

  4. Use the initial condition (0,1)(0,1)(0,1)

    Substituting x=0x=0x=0, y=1y=1y=1:

    1−1=ln⁡(1+e0)+C1-1=\ln(1+e^0)+C1−1=ln(1+e0)+C 0=ln⁡2+C0=\ln 2 + C0=ln2+C C=−ln⁡2C=-\ln 2C=−ln2

    Therefore,

    y−1y=ln⁡(1+ex)−ln⁡2=ln⁡(1+ex2)y-\frac{1}{y}=\ln(1+e^x)-\ln 2 = \ln\left(\frac{1+e^x}{2}\right)y−y1​=ln(1+ex)−ln2=ln(21+ex​)

  5. Rewrite in the form of options

    Multiply both sides by yyy:

    y2−1=yln⁡(1+ex2)y^2-1 = y\ln\left(\frac{1+e^x}{2}\right)y2−1=yln(21+ex​)

    So,

    y2=1+yln⁡(1+ex2)y^2 = 1 + y\ln\left(\frac{1+e^x}{2}\right)y2=1+yln(21+ex​)

  6. Match with options

    This is exactly Option D:

    y2=1+ylog⁡e(1+ex2)y^2 = 1 + y\log_e\left(\frac{1+e^x}{2}\right)y2=1+yloge​(21+ex​)

Therefore, the correct answer is D.

PreviousNext

More from Differential Equations

  • If x3dy + xy dx = x2dy + 2y dx; y(2) = e and x > 1, then y(4) is equal to :2020 · MCQ
  • Let y = y(x) be the solution of the differential equation, xy'- y = x2(xcosx + sinx), x > 0. if y (π) = π then y′′(2π​)+y(2π​) is equal to :2020 · MCQ
  • The solution of the differential equation dxdy​−loge​(y+3x)y+3x​+3=0 is: (where c is a constant of integration)2020 · MCQ
  • If y = y(x) is the solution of the differential equation 2+y5+ex​.dxdy​+ex=0 satisfying y(0) = 1, then a value of y(loge13) is :2020 · MCQ
  • Let y = y(x) be the solution of the differential equation cosx dxdy​+ 2ysinx = sin2x, x ∈ (0,2π​). If y (3π​) = 0, then y (4π​) is…2020 · MCQ
  • The general solution of the differential equation 1+x2+y2+x2y2​ + xy dxdy​ = 0 is : (where C is a constant of integration)2020 · MCQ
  • If y = y(x) is the solution of the differential equation, ey(dxdy​−1)=ex such that y(0) = 0, then y(1) is equal to:2020 · MCQ
  • Let y = y(x) be the solution curve of the differential equation, (y2−x)dxdy​=1, satisfying y(0) = 1. This curve intersects the x-axis at a point whose abscissa is :2020 · MCQ