Question

What kind of differential equations can be solved by direct integration?

Short answer

Question: Solve the first-order differential equation dy/dx = (2x)/(y^2) using direct integration. Answer: Step 1: Identify the Differential Equation Form The given differential equation is dy/dx = (2x)/(y^2), which is already in the form dy/dx = (1/g(y)) * (f(x)) with f(x) = 2x and g(y) = y^2. Step 2: Separate Variables Separate the variables by multiplying both sides by g(y) and dx: y^2 dy = 2x dx Step 3: Integrate Both Sides Integrate the left side with respect to y and the right side with respect to x: ∫y^2 dy = ∫2x dx Step 4: Solve for the Dependent Variable (y) Find the antiderivatives for both sides of the equation: (1/3)y^3 = x^2 + C Now, solve for y in terms of x (if possible) by taking the cube root of both sides: y(x) = (3(x^2 + C))^(1/3) This is the general solution for the given differential equation.

Step-by-step solution

Identify the Differential Equation Form

A first-order differential equation that can be solved by direct integration has the following form: dy/dx = (1/g(y)) * (f(x)) where g(y) and f(x) are continuous functions of y and x respectively.

Separate Variables

Rearrange the equation so that the terms involving x are on the right side and terms involving y are on the left side. This can be achieved by multiplying both sides by g(y) and dx: g(y) dy = f(x) dx

Integrate Both Sides

Now that the variables are separated, we can integrate both sides of the equation. Integrate the left side with respect to y, and the right side with respect to x: ∫g(y) dy = ∫f(x) dx

Solve for the Dependent Variable (y)

Find the antiderivatives for both sides of the equation and write the general solution: G(y) = F(x) + C where G(y) and F(x) are the antiderivatives of g(y) and f(x) respectively, and C is the constant of integration. Finally, solve for y in terms of x (if possible) to obtain the solution of the differential equation.

Key concepts

Direct Integration

Direct integration is a method used to solve certain types of differential equations directly by finding their antiderivatives. Unlike more complex methods, this approach is straightforward and requires the equation to be rearranged in a specific form, typically depicting a direct relationship between the derivative of a function and its variables.

In general, a differential equation suitable for direct integration is written as:

• \(\frac{dy}{dx} = h(x)\)

Upon identification, the solution involves integrating both sides with respect to their respective variables, effectively reversing the differentiation process.

This approach is particularly useful when the equation is simple and does not require conditions such as boundary values or initial conditions to be integrated. By using direct integration, we find the antiderivative of the function, capturing all potential solutions in a family of functions distinguished by a constant of integration \(C\).

Separation of Variables

Separation of variables is a powerful technique for solving certain types of first-order differential equations. This method involves manipulating the equation until all terms involving the dependent variable (usually y) are on one side of the equation, and all terms involving the independent variable (usually x) are on the other side.

The process looks like this:

• Start with an equation of the form:\(\frac{dy}{dx} = \frac{1}{g(y)} \cdot f(x)\).
• Separate variables to achieve:\(g(y) \, dy = f(x) \, dx\).

This allows us to integrate each side independently. The key benefit of this approach is the direct path it provides to the solution by ensuring that each variable is independently integrated, thus preserving the integrity of the original equation.

First-order Differential Equation

A first-order differential equation involves derivatives of a function that are of the first degree, meaning they only involve first derivatives. These equations play an essential role in modeling real-world phenomena where rates of change are dependent on a single variable at a time.

The general form of a first-order differential equation is:

• \(\frac{dy}{dx} = f(x, y)\)

Solving such equations often involves methods like direct integration or separation of variables when applicable. In more complex scenarios, other tools must be employed. First-order differential equations are foundational in calculus due to their straightforward nature and direct applicability in many fields such as physics, biology, and economics.

Continuous Functions

Continuous functions are crucial in differential equations as they ensure smoothness and integrability. A function is considered continuous if, intuitively, you can draw it without lifting your pencil off the paper. Mathematically, a function \(f(x)\) is continuous at a point \(x = a\) if the limit of the function as it approaches \(a\) equals the function's value at \(a\).

In the context of direct integration and separation of variables, why is continuity important?

• Continuous functions allow for the existence of derivatives and antiderivatives, which are critical for the integration process.
• They ensure the integration follows the fundamental theorem of calculus, connecting the processes of differentiation and integration seamlessly.
• With continuity, the solutions to differential equations are more predictable and behave consistently over their domains.

Therefore, the presence of continuous functions \(g(y)\) and \(f(x)\) is vital when employing techniques like direct integration or separation of variables.