Separation of variables, can't get y out of exponent

In summary, the concept of separation of variables is a technique used in solving differential equations by separating them into simpler equations containing only one independent variable. This is achieved by writing the given equation in the form of dy/dx = f(x)g(y) and then separating the variables on opposite sides of the equation to be solved through integration. However, in some cases, y may be present in an exponent, making it difficult to solve directly. Common mistakes when using this technique include forgetting the constant of integration and not checking for restrictions on the values of x and y. Separation of variables can be used to solve a variety of differential equations, including those involving exponential, logarithmic, and trigonometric functions.
  • #1
find_the_fun
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0
Solve the DE by using separation of variables
\(\displaystyle \frac{dy}{dx} = e^{3x+2y}\)

Break up \(\displaystyle e^{3x+2y} = e^{3x}e^{2y}\) Move x's and y's to their own side of the equation.
\(\displaystyle \frac{1}{e^{2y}} dy = e^{3x} dx\)
Integrate both sides of the equation to get \(\displaystyle \frac{-e^{2y}}{2x}=\frac{e^{3x}}{3}+C\)

I don't know how to isolate the y; I don't know how to get it down from the exponent.
 
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  • #2
find_the_fun said:
Solve the DE by using separation of variables
\(\displaystyle \frac{dy}{dx} = e^{3x+2y}\)

Break up \(\displaystyle e^{3x+2y} = e^{3x}e^{2y}\) Move x's and y's to their own side of the equation.
\(\displaystyle \frac{1}{e^{2y}} dy = e^{3x} dx\)
Integrate both sides of the equation to get \(\displaystyle \frac{-e^{2y}}{2x}=\frac{e^{3x}}{3}+C\)

I don't know how to isolate the y; I don't know how to get it down from the exponent.
I'm going to presume that the x on the LHS is a typo. Otherwise I have no idea where it came from.

Simplifying a bit we have:
\(\displaystyle e^{-2y} = -\frac{2}{3}e^{3x} - 2C\)

Your turn: Take ln of both sides. And no, it doesn't simplify beyond this, unless C = 0, which is a matter for the boundary conditions, which you don't have.

-Dan
 

FAQ: Separation of variables, can't get y out of exponent

What is the concept of separation of variables?

The concept of separation of variables is a technique used in solving differential equations. It involves separating a differential equation into simpler equations, each containing only one independent variable.

How does separation of variables work?

In separation of variables, the given differential equation is written in the form of dy/dx = f(x)g(y). The next step is to separate the variables x and y on opposite sides of the equation. This allows the equation to be solved by integrating both sides separately.

Why can't I get y out of the exponent in separation of variables?

In some cases, after separating the variables in the differential equation, one of the resulting equations may involve an exponent with y as the base. This can make it difficult to solve for y directly. In these cases, logarithmic functions can be used to isolate y and solve for it.

What are some common mistakes made when using separation of variables?

One common mistake is forgetting to include the constant of integration when solving each separated equation. Another mistake is not checking for any restrictions on the values of x and y that make the solution invalid.

What types of problems can be solved using separation of variables?

Separation of variables can be used to solve a wide range of differential equations, including both ordinary and partial differential equations. It is particularly useful for solving equations involving exponential, logarithmic, and trigonometric functions.

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