Question

\(\left(x+y^{2}\right) y^{\prime}=y\).

Short answer

The solution is implicit: \(\frac{x}{2} \ln|y| + \frac{y^2}{2} = x + C\).

Step-by-step solution

Identify the Type of Differential Equation

Notice that the equation \((x + y^2) y' = y\) is a first-order differential equation. Our task is to determine if it can be simplified into a separable form.

Rearrange the Equation

Rewrite the equation as \(y' = \frac{y}{x + y^2}\) to make it easier to identify whether it is separable.

Check for Separable Variables

The equation is separable if it can be rewritten in the form \(g(y) \, dy = f(x) \, dx\). Rearrange the equation by multiplying both sides by \((x + y^2)\ dy\): \((x + y^2)\, dy = y\, dx\).

Separate Variables

Separate the variables by rewriting the equation \((x + y^2)\, dy = y\, dx\) as \((x + y^2)\ y^{-1} \, dy = dx\).

Integrate Both Sides

Integrate the left-hand side: \(\int \frac{x}{y}\, dy + \int y\, dy\) and the right-hand side \(\int 1 \, dx\). This gives: \(\frac{x}{2} \ln|y| + \frac{y^2}{2} = x + C\).

Solve for the Constant

Since the solution involves \(C\), the general solution to the differential equation is thus given in implicit form as \(\frac{x}{2} \ln|y| + \frac{y^2}{2} = x + C\), where \(C\) is an arbitrary constant.

Key concepts

First-Order Differential Equations

First-order differential equations are equations where the highest derivative is the first derivative. In simpler terms, these equations contain a function and its first derivative, but no higher derivatives. These equations often appear in various fields such as physics, engineering, and economics. A common form of a first-order differential equation is \( \frac{dy}{dx} = f(x, y) \). This presents a relationship between the change in \( y \) with respect to \( x \), and the variables \( x \) and \( y \) themselves. Recognizing first-order equations is crucial because they have specific methods of solution. The first step in solving these is often recognizing their form and identifying if they belong to a special type, like separable or linear, which allows for certain solving techniques.

Integration Techniques

Integration techniques are essential tools for solving differential equations. Once we have separated variables in a differential equation, the next step is to integrate both sides. In the exercise, after separating variables, we encounter integrals like \( \int \frac{x}{y} \, dy + \int y \, dy \) and \( \int 1 \, dx \).

• Direct Integration: This is applicable when the integrand is straightforward. For example, \( \int 1 \, dx = x + C \).
• Integration by Substitution: This method involves simplifying the integral by substituting variables to make it easier to integrate.
• Parts: Useful for integrals involving products of functions, although it was not needed in this case.

After performing the integration, don't forget the constant \( C \), which accounts for any constant value needed to satisfy initial conditions.

Implicit Solutions

An implicit solution of a differential equation represents a relationship between \( x \) and \( y \) that is not solved for \( y \) explicitly. Instead, it's a solution in the form of an equation involving both \( x \) and \( y \). In the solution for our differential equation, we arrived at \[ \frac{x}{2} \ln|y| + \frac{y^2}{2} = x + C \] Here, \( y \) is not isolated on one side, but the relationship still holds true for any valid \( x \) and \( y \). Implicit solutions can be advantageous when it is challenging or impossible to solve explicitly for \( y \), and they still provide valuable insights into the behavior of the function.

Solving Differential Equations

Solving differential equations typically involves finding a function or set of functions that satisfy the initial equation. For the given problem, this means going from the equation \( (x + y^2) y' = y \) to a solution that expresses how \( y \) behaves relative to \( x \).

• Identify Separate Variables: Recognize if the differential equation is separable. If so, solve it by rewriting and integrating.
• Partial Fractions and Substitution: These techniques can simplify complex integrals.
• Check the Solution: It’s crucial to verify that the solution satisfies the original equation.

The process involves several steps, each building on the previous one, requiring attention to algebraic manipulation and the application of integration to solve effectively.