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Determine the general solution of the following differential equations: 4.1 dy/dx + y/(xlnx) − x = 0 4.2 d²y/dx² + 2dy/dx + 5y = 6x²

Mathematics

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Answer

y=x22−x24ln⁡x+Cln⁡xy = \frac{x^2}{2} - \frac{x^2}{4 \ln x} + \frac{C}{\ln x}

Step-by-step solution

6 steps
  1. Solve differential equation 4.1.

    The given differential equation is dydx+yxln⁡x−x=0\frac{dy}{dx} + \frac{y}{x \ln x} - x = 0.
    Rearrange it into the standard form of a first-order linear differential equation, dydx+P(x)y=Q(x)\frac{dy}{dx} + P(x)y = Q(x):

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

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

  2. Find the integrating factor I(x)I(x).

    The integrating factor is given by 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∣\int \frac{1}{u} du = \ln|u|

    Substitute back u=ln⁡xu = \ln x:

    ∫1xln⁡xdx=ln⁡∣ln⁡x∣\int \frac{1}{x \ln x} dx = \ln|\ln x|

    Assuming x>1x > 1 (so ln⁡x>0\ln x > 0), we have ln⁡(ln⁡x)\ln(\ln x).
    Now, calculate the integrating factor:

    I(x)=eln⁡(ln⁡x)=ln⁡xI(x) = e^{\ln(\ln x)} = \ln x
  3. Multiply the differential equation by the integrating factor and integrate.

    Multiply the standard form of the differential equation by I(x)I(x):

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

    The left side is the derivative of the product y⋅I(x)y \cdot I(x):

    ddx(yln⁡x)=xln⁡x\frac{d}{dx}(y \ln x) = x \ln x

    Integrate both sides with respect to xx:

    yln⁡x=∫xln⁡xdxy \ln x = \int x \ln x dx

    Use integration by parts for ∫xln⁡xdx\int x \ln x dx. Let u=ln⁡xu = \ln x and dv=xdxdv = x dx.
    Then du=1xdxdu = \frac{1}{x} dx and v=x22v = \frac{x^2}{2}.

    ∫xln⁡xdx=uv−∫vdu=(ln⁡x)(x22)−∫x22⋅1xdx\int x \ln x dx = u v - \int v du = (\ln x)\left(\frac{x^2}{2}\right) - \int \frac{x^2}{2} \cdot \frac{1}{x} dx =x22ln⁡x−∫x2dx= \frac{x^2}{2} \ln x - \int \frac{x}{2} dx =x22ln⁡x−x24+C= \frac{x^2}{2} \ln x - \frac{x^2}{4} + C

    Substitute this back into the equation for yln⁡xy \ln x:

    yln⁡x=x22ln⁡x−x24+Cy \ln x = \frac{x^2}{2} \ln x - \frac{x^2}{4} + C

    Solve for yy:

    y=x22−x24ln⁡x+Cln⁡xy = \frac{x^2}{2} - \frac{x^2}{4 \ln x} + \frac{C}{\ln x}

    The general solution for 4.1 is:

    y=x22−x24ln⁡x+Cln⁡x\boxed{y = \frac{x^2}{2} - \frac{x^2}{4 \ln x} + \frac{C}{\ln x}}
  4. Solve differential equation 4.2.

    The given differential equation is d2ydx2+2dydx+5y=6x2\frac{d^2y}{dx^2} + 2\frac{dy}{dx} + 5y = 6x^2.
    This is a second-order linear non-homogeneous differential equation. The general solution is y=yc+ypy = y_c + y_p, where ycy_c is the complementary solution and ypy_p is the particular solution.

  5. Find the complementary solution (ycy_c).

    Consider the homogeneous equation: d2ydx2+2dydx+5y=0\frac{d^2y}{dx^2} + 2\frac{dy}{dx} + 5y = 0.
    The characteristic equation is r2+2r+5=0r^2 + 2r + 5 = 0.
    Use the quadratic formula r=−b±b2−4ac2ar = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}:

    r=−2±22−4(1)(5)2(1)=−2±4−202=−2±−162=−2±4i2r = \frac{-2 \pm \sqrt{2^2 - 4(1)(5)}}{2(1)} = \frac{-2 \pm \sqrt{4 - 20}}{2} = \frac{-2 \pm \sqrt{-16}}{2} = \frac{-2 \pm 4i}{2} r=−1±2ir = -1 \pm 2i

    The roots are complex conjugates of the form α±iβ\alpha \pm i\beta, where α=−1\alpha = -1 and β=2\beta = 2.
    The complementary solution is:

    yc=eαx(C1cos⁡(βx)+C2sin⁡(βx))=e−x(C1cos⁡(2x)+C2sin⁡(2x))y_c = e^{\alpha x}(C_1 \cos(\beta x) + C_2 \sin(\beta x)) = e^{-x}(C_1 \cos(2x) + C_2 \sin(2x))
  6. Find the particular solution (ypy_p).

    The non-homogeneous term is Q(x)=6x2Q(x) = 6x^2. Since Q(x)Q(x) is a polynomial of degree 2, we assume a particular solution of the form:

    yp=Ax2+Bx+Cy_p = Ax^2 + Bx + C

    Find the first and second derivatives of ypy_p:

    yp′=2Ax+By_p' = 2Ax + B yp′′=2Ay_p'' = 2A

    Substitute ypy_p, $

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