Solving ODE Roots: y'' + 2y' + 5y = 0

  • Thread starter kasse
  • Start date
  • Tags
    Ode Roots
In summary, the conversation is discussing the computation of two roots, y1 = exp(-x)*cos2x and y2 = exp(-x)*sin2x, and their derivatives in relation to an ODE. The characteristic equation of the ODE is also mentioned, and it is concluded that the roots are correct. The conversation ends with a plan to double check the derivative of cos2x the next day.
  • #1
kasse
384
1
y'' + 2y' + 5y = 0 (*)

OK, what I have done is computing the two roots y1 = exp(-x)*cos2x and y2 = exp(-x)*sin2x.

However, when I compute the derivatives of these two, and substitute into (*), the eq. doesn't equate 0.

Are my roots wrong?
 
Physics news on Phys.org
  • #2
In that case you haven't taken the derivatives correctly, because the roots are correct.
 
  • #3
Let us first look at the characteristic equation of the ODE.

[tex]P(\lambda) = \lambda^2 + 2\lamda + 5 = 0[/tex]
[tex](\lambda + 1)^2 = -4[/tex]
[tex]\lambda + 1 = \pm2i[/tex]
[tex]\lambda = -1 \pm2i[/tex]

Your roots appear to be correct.
 
  • #4
wbclark said:
Let us first look at the characteristic equation of the ODE.

[tex]P(\lambda) = \lambda^2 + 2\lamda + 5 = 0[/tex]
[tex](\lambda + 1)^2 = -4[/tex]
[tex]\lambda + 1 = \pm2i[/tex]
[tex]\lambda = -1 \pm2i[/tex]

Your roots appear to be correct.

Yes...

The derivative of cos2x is -2sin2x, isn't it? I'm going to check this tomorrow. Now I need some sleep.
 

1. What is the meaning of "ODE" in "Solving ODE Roots: y'' + 2y' + 5y = 0"?

ODE stands for Ordinary Differential Equation. It is a mathematical equation that involves an unknown function and its derivatives. In this case, the equation y'' + 2y' + 5y = 0 is a second-order linear ODE.

2. How do you solve an ODE like y'' + 2y' + 5y = 0?

To solve this type of ODE, you can use a variety of methods such as separation of variables, variation of parameters, or using an integrating factor. However, for linear ODEs like this one, the most common method is to use the characteristic equation to find the roots and then use those roots to determine the general solution.

3. What is the significance of finding the roots of an ODE?

The roots of an ODE are the values of the independent variable (in this case, y) that make the equation true. These roots can tell us important information about the behavior of the solution to the ODE, such as where it crosses the x-axis or if it has any critical points.

4. Can an ODE have complex roots?

Yes, an ODE can have complex roots. In fact, complex roots are often present in second-order linear ODEs with constant coefficients, like y'' + 2y' + 5y = 0. In these cases, the general solution will involve complex numbers and the solution will have both real and imaginary components.

5. How do you find the roots of an ODE like y'' + 2y' + 5y = 0?

To find the roots of an ODE, you first need to rearrange the equation into the standard form y'' + py' + qy = 0, where p and q are constants. Then, you can use the characteristic equation, which is r^2 + pr + q = 0, to find the roots. In this case, the roots will be complex numbers, given by the formula r = (-p ± √(p^2 - 4q)) / 2. Once you have the roots, you can use them to determine the general solution to the ODE.

Similar threads

  • Calculus and Beyond Homework Help
Replies
7
Views
500
Replies
4
Views
499
  • Calculus and Beyond Homework Help
Replies
17
Views
2K
  • Calculus and Beyond Homework Help
Replies
9
Views
1K
  • Calculus and Beyond Homework Help
Replies
3
Views
273
  • Calculus and Beyond Homework Help
Replies
3
Views
570
  • Calculus and Beyond Homework Help
Replies
9
Views
1K
  • Calculus and Beyond Homework Help
Replies
11
Views
743
  • Calculus and Beyond Homework Help
Replies
1
Views
963
Replies
5
Views
1K
Back
Top