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Differential Equations question

2011 · Shift 0 · Q37
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Differential Equations question

2011 · Shift 0 · Q37

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
If dydx=y+3>0  {{dy} \over {dx}} = y + 3 \gt 0\,\,dxdy​=y+3>0 and y(0)=2,y(0)=2,y(0)=2, then y(ln⁡2)y\left( {\ln 2} \right)y(ln2) is equal to :
  1. A
    555
  2. B
    131313
  3. C
    −2-2−2
  4. D
    777
View written solutionFree

Correct answer: D

  1. Given differential equation

    dydx=y+3,y(0)=2\frac{dy}{dx}=y+3, \qquad y(0)=2dxdy​=y+3,y(0)=2

    We need to find:

    y(ln⁡2)y(\ln 2)y(ln2)

  2. Rewrite in standard linear form

    dydx−y=3\frac{dy}{dx}-y=3dxdy​−y=3

    This is a first-order linear differential equation.

  3. Find the integrating factor

    For

    dydx+Py=Q,\frac{dy}{dx}+Py=Q,dxdy​+Py=Q,

    the integrating factor is

    IF=e∫P dxIF=e^{\int P\,dx}IF=e∫Pdx

    Here, P=−1P=-1P=−1, so

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

  4. Multiply the equation by the integrating factor

    e−xdydx−e−xy=3e−xe^{-x}\frac{dy}{dx}-e^{-x}y=3e^{-x}e−xdxdy​−e−xy=3e−x

    The left side becomes:

    ddx(ye−x)=3e−x\frac{d}{dx}(ye^{-x})=3e^{-x}dxd​(ye−x)=3e−x

  5. Integrate both sides

    ye−x=∫3e−x dx=−3e−x+Cye^{-x}=\int 3e^{-x}\,dx=-3e^{-x}+Cye−x=∫3e−xdx=−3e−x+C

    Multiply by exe^xex:

    y=−3+Cexy=-3+Ce^xy=−3+Cex

  6. Use the initial condition

    Given y(0)=2y(0)=2y(0)=2:

    2=−3+Ce0=−3+C2=-3+Ce^0=-3+C2=−3+Ce0=−3+C

    C=5C=5C=5

    So the solution is:

    y=5ex−3y=5e^x-3y=5ex−3

  7. Compute y(ln⁡2)y(\ln 2)y(ln2)

    y(ln⁡2)=5eln⁡2−3=5⋅2−3=10−3=7y(\ln 2)=5e^{\ln 2}-3=5\cdot 2-3=10-3=7y(ln2)=5eln2−3=5⋅2−3=10−3=7

  8. Compare with options

    The correct option is:

    D:7\boxed{D: 7}D:7​

  9. Compare with stored correct answer

    Stored correct answer: D

    Our derived answer: D

    Hence, they agree.

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