Bernoulli Equation 86A652
1. **State the problem:** Solve the differential equation $$xy' + y = x^4 y^2$$ with the initial condition $$y(1) = 1$$.
2. **Rewrite the equation:** Divide both sides by $$x$$ (assuming $$x \neq 0$$) to get
$$y' + \frac{y}{x} = x^3 y^2$$.
3. **Identify the type:** This is a Bernoulli differential equation of the form
$$y' + P(x) y = Q(x) y^n$$ where $$P(x) = \frac{1}{x}$$, $$Q(x) = x^3$$, and $$n = 2$$.
4. **Substitution:** Let $$v = y^{1-n} = y^{-1}$$. Then,
$$v' = -y^{-2} y'$$.
5. **Rewrite the original equation in terms of $$v$$:**
From $$y' + \frac{y}{x} = x^3 y^2$$,
multiply both sides by $$-y^{-2}$$:
$$-y^{-2} y' - \frac{y^{-1}}{x} = -x^3$$,
which becomes
$$v' - \frac{v}{x} = -x^3$$.
6. **Solve the linear differential equation for $$v$$:**
The integrating factor is
$$\mu(x) = e^{-\int \frac{1}{x} dx} = e^{-\ln|x|} = x^{-1}$$.
Multiply both sides by $$x^{-1}$$:
$$x^{-1} v' - x^{-2} v = -x^{2}$$,
which is
$$(x^{-1} v)' = -x^{2}$$.
7. **Integrate both sides:**
$$\int (x^{-1} v)' dx = \int -x^{2} dx$$
$$x^{-1} v = -\frac{x^{3}}{3} + C$$.
8. **Solve for $$v$$:**
$$v = -\frac{x^{4}}{3} + C x$$.
9. **Recall substitution:**
$$v = y^{-1}$$, so
$$y = \frac{1}{v} = \frac{1}{-\frac{x^{4}}{3} + C x} = \frac{1}{C x - \frac{x^{4}}{3}}$$.
10. **Apply initial condition $$y(1) = 1$$:**
$$1 = \frac{1}{C \cdot 1 - \frac{1^{4}}{3}} = \frac{1}{C - \frac{1}{3}}$$,
which implies
$$C - \frac{1}{3} = 1 \Rightarrow C = \frac{4}{3}$$.
11. **Final solution:**
$$y = \frac{1}{\frac{4}{3} x - \frac{x^{4}}{3}} = \frac{3}{4x - x^{4}}$$.
**Answer:** $$\boxed{y = \frac{3}{4x - x^{4}}}$$