# Help in solving a second-order linear differential equation

• Aero
In summary, the conversation discusses how to solve the equation \frac{{d^2 y}}{{dx^2 }} + \left( {Ax + B} \right)y = 0 through a change of variable and the use of the Airy equation. It is noted that there are three distinct solutions to the equation, one real-valued and two complex. Additional resources for the Airy function are recommended for further understanding.
Aero
$$\frac{{d^2 y}}{{dx^2 }} + \left( {Ax + B} \right)y = 0$$

I have tried lots of substitions, but a solution won't pop out. Can anyone help solve this?

Thanks.

Make the change of variable

$$Ax + B = \lambda u$$

(lambda is a constant) this will give you

$$\frac{A^2}{\lambda^2} \frac{d^2 y}{d u^2} + \lambda u y = 0$$

so if you then set

$$\lambda = -A^{\frac{2}{3}}$$

you then have

$$\frac{d^2 y}{d u^2} - u y = 0$$

which is the Airy equation (in u). Have a look on Wikipedia or elsewhere on Airy functions and such - or just type in "Airy Equation".

edit: note you will actually get three different solutions as the condition for lambda is

$$\frac{\lambda^3}{A^2} = -1$$

which means that there are three values of lambda that satisfy this (i.e. three distinct cube-roots) - one will be real-valued (already given) plus two complex ones.

Here is a link for the Airy function

http://mathworld.wolfram.com/AiryFunctions.html

Last edited:

Sure, I'd be happy to help with solving this second-order linear differential equation. The first step in solving any differential equation is to identify the type of equation and its order. In this case, we have a second-order linear differential equation, which means it has the form:

\frac{{d^2 y}}{{dx^2 }} + P(x)\frac{{dy}}{{dx}} + Q(x)y = R(x)

where P(x) and Q(x) are functions of x and R(x) is a function of y.

The next step is to check if the equation is homogeneous or non-homogeneous. A homogeneous equation has a R(x) term equal to 0, while a non-homogeneous equation has a non-zero R(x) term. In this case, our equation is homogeneous since R(x) = 0.

To solve a homogeneous second-order linear differential equation, we use the method of undetermined coefficients. This involves assuming a solution of the form y = e^{mx}, where m is a constant. We then substitute this into the equation and solve for m.

In our case, the equation becomes:

m^2e^{mx} + (Ax+B)e^{mx} = 0

We can simplify this to:

(m^2 + Am + B)e^{mx} = 0

Since e^{mx} is never equal to 0, the only way for this equation to hold is if the coefficient (m^2 + Am + B) is equal to 0. This gives us a quadratic equation in terms of m, which we can solve to find the values of m.

Once we have the values of m, we can then use them to find the general solution of the differential equation, which has the form:

y = c_1e^{m_1x} + c_2e^{m_2x}

where c_1 and c_2 are constants determined by any initial conditions given in the problem.

I hope this helps in solving your second-order linear differential equation. If you have any further questions or need clarification on any steps, please don't hesitate to ask. Good luck!

## What is a second-order linear differential equation?

A second-order linear differential equation is a mathematical equation that involves a function and its first and second derivatives. It can be written in the form of a2(x)y'' + a1(x)y' + a0(x)y = f(x), where a2, a1, and a0 are coefficients and f(x) is a function of x.

## What is the process for solving a second-order linear differential equation?

The process for solving a second-order linear differential equation involves finding the general solution, which is a family of solutions that satisfies the equation for all values of the coefficients. This can be done by using various techniques such as the method of undetermined coefficients or the method of variation of parameters.

## What is the difference between a homogeneous and non-homogeneous second-order linear differential equation?

A homogeneous second-order linear differential equation has a right-hand side of f(x) = 0, meaning there is no external force acting on the system. A non-homogeneous second-order linear differential equation has a non-zero right-hand side, representing an external force or input.

## How can I check if my solution to a second-order linear differential equation is correct?

To check if your solution is correct, you can substitute it into the equation and see if it satisfies the equation for all values of x. You can also take the second derivative of your solution and see if it matches the second derivative in the original equation.

## What are some applications of second-order linear differential equations?

Second-order linear differential equations are commonly used in physics and engineering to model various systems such as oscillations, circuits, and heat transfer. They are also used in economics and finance to model growth and decay processes.

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