Respuesta :
[tex]y'-\dfrac5xy=\dfrac{y^5}{x^9}[/tex]
Divide through both sides by [tex]y^5[/tex]:
[tex]y^{-5}y'-\dfrac5xy^{-4}=x^{-9}[/tex]
Now let [tex]z=y^{-4}[/tex], so that [tex]z'=-4y^{-5}y'[/tex]. Then
[tex]-\dfrac{z'}4-\dfrac5xz=x^{-9}[/tex]
[tex]z'+\dfrac{20}xz=-4x^{-9}[/tex]
Multiply both sides by [tex]x^{20}[/tex]:
[tex]x^{20}z'+20x^{19}z=\left(x^{20}z\right)'=-4x^{11}[/tex]
Integrate both sides to get
[tex]x^{20}z=-\dfrac{x^{12}}3+C\implies z=-\dfrac1{3x^8}+\dfrac C{x^{20}}[/tex]
Solve for [tex]y[/tex]:
[tex]y=\left(\dfrac C{x^{20}}-\dfrac1{3x^8}\right)^{-1/4}[/tex]
Given that [tex]y(1)=1[/tex], we have
[tex]1=\left(C-\dfrac13\right)^{-1/4}\implies C=\dfrac43[/tex]
so the particular solution is
[tex]y=\left(\dfrac4{3x^{20}}-\dfrac1{3x^8}\right)^{-1/4}[/tex]
which we can rewrite as
[tex]\boxed{y=\dfrac{3^{1/4}}{x^5\left(4-x^{12}\right)^{1/4}}}[/tex]
The solution to the differential equation y′−5/x*y=y^5/x^9, by using the appropriate substitution is [tex]\mathbf{y^{-4} = \dfrac{1}{3}x^{-8} + \dfrac{4}{3}x^{-20}}[/tex] . The general solution that
satisfies y(1) = 1 is [tex]\mathbf{y(x) = \Big (\dfrac{3x^{20}}{4-x^{12}} \Big) ^{1/4}}[/tex]
The given equation is well expressed as:
[tex]\mathbf{y' - \dfrac{5}{x}y =\dfrac{y^5}{y^9} \ , \ y(1) = 1}[/tex]
We are to apply the Bernoulli equation approach to solve the given differential equation.
So, the first step is to divide the differential equation by [tex]\mathbf{y^5}[/tex], by doing so, we have
[tex]\mathbf{y^{-5}y' - \dfrac{5}{x}y^{-4} =\dfrac{1}{y^9} }[/tex]
Now, let make:
- [tex]\mathbf{y^{-4} = u}[/tex] and;
- [tex]\mathbf{-4y^{-5} \dfrac{dy}{dx}= \dfrac{du}{dx}}[/tex]
∴
From the equation [tex]\mathbf{y^{-5}y' - \dfrac{5}{x}y^{-4} =\dfrac{1}{y^9} }[/tex] ;
- [tex]\mathbf{y^{-5} \dfrac{dy}{dx}= -\dfrac{1}{4} \dfrac{du}{dx}}[/tex]
We can now have the equation to be differentiated as:
- [tex]\mathbf{-\dfrac{1}{4}\dfrac{du}{dx} - \dfrac{5}{x}u = \dfrac{1}{x^9}}[/tex]
- Multiplying both sides by 4, we have:
- [tex]\mathbf{\dfrac{du}{dx} + \dfrac{20}{x}u = -\dfrac{4}{x^9}}[/tex]
Hence, we can infer that the linear equation in terms of u is:
[tex]\mathbf{\dfrac{du}{dx} + \dfrac{20}{x}u = -\dfrac{4}{x^9}}[/tex]
Now, taking the integration factor: [tex]\mathbf{\mu(x) = e^ {^{\Big \int \frac{20}{x}\ dx }} = e^{20 \ In \ x} = x^{20} }[/tex]
We need to multiply the integration factor [tex]\mathbf{ \mu(x)}[/tex] with the equation
[tex]\mathbf{\dfrac{du}{dx} + \dfrac{20}{x}u = -\dfrac{4}{x^9}}[/tex] , so we have:
- [tex]\mathbf{x^{20} \Big(\dfrac{du}{dx} + \dfrac{20}{x}u = -\dfrac{4}{x^9}(x^{20} )\Big) }[/tex]
- [tex]\mathbf{x^{20} \dfrac{du}{dx} + 20x^{19}u = -4x^{11}} }[/tex]
Taking the integral of both sides;
- [tex]\mathbf{\int (x^{20} u)' dx = \int -4x^{11} \ dx }[/tex]
- [tex]\mathbf{ x^{20} u= -4\Big ( \dfrac{x^{12} }{12} \Big ) +c }[/tex]
- [tex]\mathbf{ x^{20} u= -\dfrac{1}{3}x^{12} +c }[/tex]
- [tex]\mathbf{u = -\dfrac{1}{3} x^{-8} +cx ^{-20}}[/tex]
Recall that: [tex]\mathbf{u = y^{-4}}[/tex], if we replace the value of u with [tex]\mathbf{ y^{-4}}[/tex] in the above equation, we will have:
- [tex]\mathbf{ y^{-4} = -\dfrac{1}{3}x^{-8} +cx ^{-20}}[/tex]
Using the given initial condition y(1) = 1 to the equation above to determine the value of c, we have:
- [tex]\mathbf{\dfrac{1}{y^4(1)} = -\dfrac{1}{3}(1)^{-8} + c(1) ^{-20} }[/tex]
- [tex]\mathbf{\dfrac{1}{1} = -\dfrac{1}{3}+c}[/tex]
- [tex]\mathbf{c = \dfrac{4}{3}}[/tex]
∴
[tex]\mathbf{y^{-4} = -\dfrac{1}{3} x^{-8} + \dfrac{4}{3}x^{-20}}[/tex]
Now, to solve for y(x); we have:
- [tex]\mathbf{\dfrac{1}{y^{4} }= -\dfrac{1}{3x^8} + \dfrac{4/3}{x^{20}}}}[/tex]
- [tex]\mathbf{\dfrac{1}{y^4}= \dfrac{1}{3x^8}\Big(-1 + \dfrac{4}{x^{12}} \Big)} }[/tex]
- [tex]\mathbf{\dfrac{1}{y^4} = \dfrac{4-x^{12}}{3x^{20}}}[/tex]
- [tex]\mathbf{y^4 = \dfrac{3x^{20}}{4-x^{12}}}[/tex]
- [tex]\mathbf{y =\Big( \dfrac{3x^{20}}{4-x^{12} }\Big) ^{1/4}}[/tex]
Therefore, the general solution that satisfies y(1) = 1 is [tex]\mathbf{y(x) = \Big (\dfrac{3x^{20}}{4-x^{12}} \Big) ^{1/4}}[/tex]
Learn more about Bernoulli differential equation here:
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