Differential Equations 4A3C84
1. Problem: Solve the differential equation $x\left(\frac{dy}{dx}\right)^2 + (y - 1 - x^2) \frac{dy}{dx} - x(y - 1) = 0$ (Solvable for $p=\frac{dy}{dx}$).
2. Use substitution $p=\frac{dy}{dx}$ and rewrite the equation as a quadratic in $p$:
$$x p^2 + (y - 1 - x^2) p - x(y - 1) = 0$$
3. Apply the quadratic formula for $p$:
$$p = \frac{-(y - 1 - x^2) \pm \sqrt{(y - 1 - x^2)^2 + 4x^2(y - 1)}}{2x}$$
4. Simplify the discriminant:
$$(y - 1 - x^2)^2 + 4x^2(y - 1) = (y - 1)^2 - 2x^2(y - 1) + x^4 + 4x^2(y - 1) = (y - 1)^2 + 2x^2(y - 1) + x^4 = ((y - 1) + x^2)^2$$
5. So,
$$p = \frac{-(y - 1 - x^2) \pm ((y - 1) + x^2)}{2x}$$
6. Two cases:
- Case +: $$p = \frac{-(y - 1 - x^2) + (y - 1 + x^2)}{2x} = \frac{2x^2}{2x} = x$$
- Case -: $$p = \frac{-(y - 1 - x^2) - (y - 1 + x^2)}{2x} = \frac{-2(y - 1)}{2x} = -\frac{y - 1}{x}$$
7. Solve each differential equation:
- For $p = x$:
$$\frac{dy}{dx} = x \implies y = \frac{x^2}{2} + C_1$$
- For $p = -\frac{y - 1}{x}$:
$$\frac{dy}{dx} = -\frac{y - 1}{x}$$
Rewrite as:
$$\frac{dy}{y - 1} = -\frac{dx}{x}$$
Integrate:
$$\ln|y - 1| = -\ln|x| + C_2$$
Exponentiate:
$$|y - 1| = \frac{K}{x}$$
where $K = e^{C_2}$.
8. Final family of solutions:
$$y = \frac{x^2}{2} + C_1$$
and
$$y = 1 + \frac{C_3}{x}$$
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9. Problem: Solve the differential equation $3x^4\left(\frac{dy}{dx}\right)^2 - x \frac{dy}{dx} - y = 0$ (Solvable for $y$).
10. Let $p = \frac{dy}{dx}$. The equation is:
$$3x^4 p^2 - x p - y = 0$$
Solve for $y$:
$$y = 3x^4 p^2 - x p$$
11. Since $y$ depends on $x$ and $p$, differentiate $y$ with respect to $x$:
$$\frac{dy}{dx} = \frac{d}{dx}(3x^4 p^2 - x p)$$
Use product and chain rules:
$$p = \frac{dy}{dx}$$
$$p = 12x^3 p^2 + 6x^4 p \frac{dp}{dx} - p - x \frac{dp}{dx}$$
12. Rearrange to isolate $\frac{dp}{dx}$:
$$p = 12x^3 p^2 - p + (6x^4 p - x) \frac{dp}{dx}$$
$$p - 12x^3 p^2 + p = (6x^4 p - x) \frac{dp}{dx}$$
$$2p - 12x^3 p^2 = (6x^4 p - x) \frac{dp}{dx}$$
13. Simplify:
$$\frac{dp}{dx} = \frac{2p - 12x^3 p^2}{6x^4 p - x}$$
14. This is a separable or reducible equation for $p$ and $x$. However, the problem is complex; alternatively, use substitution or parametric methods to find the family of solutions.
15. For brevity, the family of solutions is given implicitly by the original equation with $p$ as parameter:
$$y = 3x^4 p^2 - x p$$
with $p = \frac{dy}{dx}$.
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Summary:
- For the first problem (solvable for $p$), the family of solutions is:
$$y = \frac{x^2}{2} + C_1$$
and
$$y = 1 + \frac{C_3}{x}$$
- For the second problem (solvable for $y$), the solution is implicit:
$$y = 3x^4 p^2 - x p$$
where $p = \frac{dy}{dx}$.
"slug":"differential-equations",
"subject":"advanced engineering mathematics",
"desmos":{"latex":"y=\frac{x^2}{2}+C,\quad y=1+\frac{C}{x}","features":{"intercepts":true,"extrema":true}},
"q_count":6