Question

Determine whether the following equations are separable. If so, solve the initial value problem. $$\frac{d y}{d x}=e^{x-y}, y(0)=\ln 3$$

Short answer

Given: \(\frac{dy}{dx} = e^{x-y}\) Initial condition: \(y(0) = \ln 3\) Solution: The given differential equation is separable, and the solution to the initial value problem is \(e^y = e^x + 2\).

Step-by-step solution

Testing for separability

To test if the given equation, \(\frac{dy}{dx} = e^{x-y}\), is separable, we want to rewrite it as \((f(x) dx)(g(y) dy)\). Concentrate on the right side of the equation and multiply both sides by \(e^y\) to bring the y terms on one side and divide by \(e^x\) to keep the x terms on the other side. $$e^y \frac{dy}{dx} = e^x.$$

Rewriting the equation to show separability

Now that we have established separability, we rewrite the equation clearly in the product form to make it explicitly separable: $$e^y dy= e^x dx$$

Integrate both sides of the equation

Integrate both sides of the equation to isolate the functions of x and y. Don't forget to include the constants of integration. $$\int e^y dy = \int e^x dx + C$$ $$e^y = e^x + C$$

Apply the initial condition to find C

Given that \(y(0) = \ln 3\), plug the values x = 0 and y = ln 3 into the solution equation to find C: $$e^{\ln 3} = e^0 + C$$ $$3 = 1 + C$$ So, C = 2.

Write the final solution

Replace C with its value in the solution equation to obtain the final solution in terms of x and y. $$e^y = e^x + 2$$ Thus, the solution to the initial value problem is \(e^y = e^x + 2\).

Key concepts

Initial Value Problem

Imagine you are given a starting point, much like getting a head start in a race. This is basically what an Initial Value Problem (IVP) provides in the world of differential equations. An IVP doesn't just give you an ordinary differential equation (ODE) to solve. It also hands you a specific point, a concrete starting condition to anchor your solution.

If your equation is like a road map, then the initial condition tells you exactly where to begin. So, when dealing with our example of the differential equation \(\frac{dy}{dx} = e^{x-y}\) with the initial condition \(y(0) = \ln 3\), it clarifies that at \(x = 0\), \(y\) must be \(\ln 3\).
This starting point allows you to find a unique solution, which is like charting a specific course instead of wandering on different paths. This is crucial because without it, there could be infinitely many paths (solutions), but the initial value ensures we follow just one.

Integrating Functions

Integration can be thought of as the reverse process of differentiation. If differentiation is all about breaking things down into smaller rates and changes, integration builds them back up.
In our task, after proving that the equation is separable—by isolating the \(y\) terms from the \(x\) terms—the problem transforms into integrating both sides separately.

That's why we see equations like \(\int e^y dy = \int e^x dx\).
Integrating the function \(e^y\) with respect to \(y\) reverses the derivative operation of exponential functions, while integrating \(e^x\) with respect to \(x\) does the same for \(x\). This step is the bridge that translates the separated terms into solutions.

• For \(e^y dy\), integration results in \(e^y\).
• Similarly, \(e^x dx\) when integrated, becomes \(e^x\).

The result is a harmonious balance—a nice equation like \(e^y = e^x + C\)—showing that integration helps reconcile and unify different parts of the equation.

Constants of Integration

Once you integrate both sides of an equation, a crucial part of the resulting expression is the constant of integration, represented by \(C\). In mathematics, this constant emerges because indefinite integration in calculus leaves a degree of freedom—there are limitless numbers of predicted outcomes.

To put it simply, whenever you integrate indefinitely, you're effectively saying, "I know the shape of the function, but I’m not quite sure of its exact position on the graph."
This is where your initial conditions come into play.

Returning to our solution, we had \(e^y = e^x + C\). To determine exactly what \(C\) is, the initial condition \(y(0) = \ln 3\) is hugely helpful. By substituting \(x = 0\) and \(y = \ln 3\) into the equation, we find that \(C = 2\).

• The magic of the constant \(C\) is that it captures that elusive piece of information defining the precise path of our solution.
• Without finding \(C\), we have a family of solutions. With it, we have one specific answer.

This consistent integration bonus helps ensure that your final solution is concrete and consistent with the world of differential equations!