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Calculate the particular solution of dy/dx - y/(x ln x) = 1/x at (2; 0). Determine the general solution of d2y/dx2 + dy/dx − 6y = 2e⁻³x.

Mathematics

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2 parts
4.1

y=ln⁡xln⁡(ln⁡xln⁡2)y = \ln x \ln\left(\frac{\ln x}{\ln 2}\right)

4.2

y=C1e−3x+C2e2x−25xe−3xy = C_1 e^{-3x} + C_2 e^{2x} - \frac{2}{5} x e^{-3x}

Step-by-step solution

7 steps

Here are the solutions to the given differential equations.

Part 4.1· 4 steps

Calculate the particular solution of dydx−yxln⁡x=1x\frac{dy}{dx} - \frac{y}{x \ln x} = \frac{1}{x} at (2,0)(2, 0).

  1. Rewrite the differential equation in the standard form for a first-order linear differential equation, dydx+P(x)y=Q(x)\frac{dy}{dx} + P(x)y = Q(x).

    dydx+(−1xln⁡x)y=1x\frac{dy}{dx} + \left(-\frac{1}{x \ln x}\right)y = \frac{1}{x}

    Here, P(x)=−1xln⁡xP(x) = -\frac{1}{x \ln x} and Q(x)=1xQ(x) = \frac{1}{x}.

  2. Find the integrating factor, I(x)=e∫P(x)dxI(x) = e^{\int P(x) dx}.

    First, calculate ∫P(x)dx\int P(x) dx:

    ∫−1xln⁡xdx\int -\frac{1}{x \ln x} dx

    Let u=ln⁡xu = \ln x, then du=1xdxdu = \frac{1}{x} dx.

    ∫−1udu=−ln⁡∣u∣=−ln⁡∣ln⁡x∣\int -\frac{1}{u} du = -\ln|u| = -\ln|\ln x|

    Now, find the integrating factor:

    I(x)=e−ln⁡∣ln⁡x∣=eln⁡((ln⁡x)−1)=1ln⁡xI(x) = e^{-\ln|\ln x|} = e^{\ln((\ln x)^{-1})} = \frac{1}{\ln x}

    (Assuming x>1x>1 for ln⁡x\ln x to be positive and defined, which is true for x=2x=2).

  3. Multiply the differential equation by the integrating factor and integrate.

    The left side becomes ddx(y⋅I(x))\frac{d}{dx}(y \cdot I(x)).

    ddx(y⋅1ln⁡x)=1x⋅1ln⁡x\frac{d}{dx}\left(y \cdot \frac{1}{\ln x}\right) = \frac{1}{x} \cdot \frac{1}{\ln x} ddx(yln⁡x)=1xln⁡x\frac{d}{dx}\left(\frac{y}{\ln x}\right) = \frac{1}{x \ln x}

    Integrate both sides with respect to xx:

    ∫ddx(yln⁡x)dx=∫1xln⁡xdx\int \frac{d}{dx}\left(\frac{y}{\ln x}\right) dx = \int \frac{1}{x \ln x} dx yln⁡x=ln⁡∣ln⁡x∣+C\frac{y}{\ln x} = \ln|\ln x| + C

    The general solution is y=ln⁡x(ln⁡∣ln⁡x∣+C)y = \ln x (\ln|\ln x| + C).

  4. Use the initial condition $(2, 0)$ to find the particular solution.

    Substitute x=2x=2 and y=0y=0 into the general solution:

    0=ln⁡2(ln⁡∣ln⁡2∣+C)0 = \ln 2 (\ln|\ln 2| + C)

    Since ln⁡2≠0\ln 2 \neq 0, we must have:

    ln⁡∣ln⁡2∣+C=0\ln|\ln 2| + C = 0 C=−ln⁡∣ln⁡2∣C = -\ln|\ln 2|

    Substitute the value of CC back into the general solution:

    y=ln⁡x(ln⁡∣ln⁡x∣−ln⁡∣ln⁡2∣)y = \ln x (\ln|\ln x| - \ln|\ln 2|)

    Using the logarithm property ln⁡a−ln⁡b=ln⁡(ab)\ln a - \ln b = \ln\left(\frac{a}{b}\right):

    y=ln⁡x(ln⁡∣ln⁡xln⁡2∣)y = \ln x \left( \ln \left| \frac{\ln x}{\ln 2} \right| \right)

    The particular solution is:

    y=ln⁡xln⁡(ln⁡xln⁡2)\boxed{y = \ln x \ln\left(\frac{\ln x}{\ln 2}\right)}

Part 4.2

y=C1e−3x+C2e2x−25xe−3xy = C_1 e^{-3x} + C_2 e^{2x} - \frac{2}{5} x e^{-3x}

Determine the general solution of d2ydx2+dydx−6y=2e−3x\frac{d^2 y}{dx^2} + \frac{dy}{dx} - 6y = 2e^{-3x}.

  1. Find the complementary solution ycy_c by solving the homogeneous equation d2ydx2+dydx−6y=0\frac{d^2 y}{dx^2} + \frac{dy}{dx} - 6y = 0.

    The characteristic equation is:

    r2+r−6=0r^2 + r - 6 = 0

    Factor the quadratic equation:

    (r+3)(r−2)=0(r+3)(r-2) = 0

    The roots are r1=−3r_1 = -3 and r2=2r_2 = 2.
    The complementary solution is:

    yc=C1e−3x+C2e2xy_c = C_1 e^{-3x} + C_2 e^{2x}
  2. Find the particular solution ypy_p using the method of undetermined coefficients.

    The right-hand side is g(x)=2e−3xg(x) = 2e^{-3x}. Since −3-3 is a root of the characteristic equation (and it's a simple root), we must multiply the standard guess by xx.
    Let yp=Axe−3xy_p = Ax e^{-3x}.
    Calculate the first and second derivatives of ypy_p:

    yp′=Ae−3x+Ax(−3e−3x)=Ae−3x−3Axe−3x=A(1−3x)e−3xy_p' = A e^{-3x} + Ax(-3e^{-3x}) = A e^{-3x} - 3Ax e^{-3x} = A(1-3x)e^{-3x} yp′′=A(−3e−3x)+(−3Ae−3x−3Ax(−3e−3x))=−3Ae−3x−3Ae−3x+9Axe−3x=A(−6+9x)e−3xy_p'' = A(-3e^{-3x}) + (-3A e^{-3x} - 3Ax(-3e^{-3x})) = -3A e^{-3x} - 3A e^{-3x} + 9Ax e^{-3x} = A(-6+9x)e^{-3x}

    Substitute ypy_p, yp′y_p', and yp′′y_p'' into the non-homogeneous differential equation:

    A(−6+9x)e−3x+A(1−3x)e−3x−6(Axe−3x)=2e−3xA(-6+9x)e^{-3x} + A(1-3x)e^{-3x} - 6(Ax e^{-3x}) = 2e^{-3x}

    Divide by e−3xe^{-3x}:

    A(−6+9x)+A(1−3x)−6Ax=2A(-6+9x) + A(1-3x) - 6Ax = 2 −6A+9Ax+A−3Ax−6Ax=2-6A + 9Ax + A - 3Ax - 6Ax = 2

    Combine like terms:

    (−6A+A)+(9Ax−3Ax−6Ax)=2(-6A + A) + (9Ax - 3Ax - 6Ax) = 2 −5A+0x=2-5A + 0x = 2 −5A=2-5A = 2 A=−25A = -\frac{2}{5}

    So, the particular solution is:

    yp=−25xe−3xy_p = -\frac{2}{5} x e^{-3x}
  3. Write the general solution y=yc+ypy = y_c + y_p.

    y=C1e−3x+C2e2x−25xe−3xy = C_1 e^{-3x} + C_2 e^{2x} - \frac{2}{5} x e^{-3x}

    The general solution is:

    y=C1e−3x+C2e2x−25xe−3x\boxed{y = C_1 e^{-3x} + C_2 e^{2x} - \frac{2}{5} x e^{-3x}}
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