What is Variation of parameters: Definition and 116 Discussions

In mathematics, variation of parameters, also known as variation of constants, is a general method to solve inhomogeneous linear ordinary differential equations.
For first-order inhomogeneous linear differential equations it is usually possible to find solutions via integrating factors or undetermined coefficients with considerably less effort, although those methods leverage heuristics that involve guessing and do not work for all inhomogeneous linear differential equations.
Variation of parameters extends to linear partial differential equations as well, specifically to inhomogeneous problems for linear evolution equations like the heat equation, wave equation, and vibrating plate equation. In this setting, the method is more often known as Duhamel's principle, named after Jean-Marie Duhamel (1797–1872) who first applied the method to solve the inhomogeneous heat equation. Sometimes variation of parameters itself is called Duhamel's principle and vice versa.

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1. Variation of parameter VS Undetermined Coefficients

TL;DR Summary: Variation of parameter VS Undetermined Coefficients Hi all, Suppose we want to solve the following ODE 2y''+y'-y=x+7 with two different methods: undetermined coefficients and variation of parameters. The solutions to the homogeneous problem are given by y_1(x)=exp(-x) and...
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A Variation of parameters, Green's functions, Wronskian

Hi PF! I am trying to solve an ODE by casting it as an operator problem, say ##K[y(x)] = \lambda M[y(x)]##, where ##y## is a trial function, ##x## is the independent variable, ##\lambda## is the eigenvalue, and ##K,M## are linear differential operators. For this particular problem, it's easier...
3. ODE question: Understanding a step in the solution

Homework Statement Hi there, I don't nee help with solving a question, so much as understanding a step in the provided worked solution. It's using variation of parameters to solve the ode y''+ y = g(t). I've attached the steps in the picture file, and the bit after the word 'Now' what are they...
4. Variation of Parameters to solve a second order ODE

Homework Statement The question I am working on is the one in the file attached. Homework Equations y = u1y1 + u2y2 : u1'y1 + u2'y2 = 0 u1'y1' + u2'y2' = g(t) The Attempt at a Solution I think I have got part (i) completed, with y1 = e3it and y2 = e-3it. This gives a general solution to the...
5. What is the general solution for a DE involving cosh and sinh?

Homework Statement Solve the DE by variation of parameters: y'' - y = cosh(x) Homework EquationsThe Attempt at a Solution I got m = 1 and m = -1 so y = c_1e^x + c_2e^{-x} + y_p y_p = u_1e^x + u_2e^{-x} The wonksian gave me -2 so u_1' = \frac{\begin{vmatrix} 0 & e^{-x} \\ cosh(x)...
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25. Finding general solution to Euler equation via variation of parameters

Homework Statement The problem is attached as TheProblemAndSolution.png, and everything is typewritten, so it should be easily legible (but you will likely need to zoom into read the text since the image's height is significantly larger than its width). Homework Equations Differential...
26. Variation of parameters- 2nd order linear equation

Homework Statement solve 4y''-4y'+y=16et/2 Homework Equations v1= -∫ y2g/w v2= ∫ y1g/w The Attempt at a Solution http://imgur.com/gxXlfdH the correct answer is 2t^2 e^(t/2) instead of what i have though, i am not sure what i am doing wrong?
27. Y'' + y = f(x) - Variation of Parameters?

y'' + y = f(x) -- Variation of Parameters? Homework Statement Use variation of parameters to solve ##y'' + y = f(x), y(0) = y'(0) = 0.##Homework Equations A description of the method is here: http://en.wikipedia.org/wiki/Variation_of_parametersThe Attempt at a Solution The complementary...
28. MHB What is the general solution for a differential equation with a secant term?

I was given the problem, "Find the general solution of the given differential equation." $$y^{\prime\prime}+9y=9\sec^2(3t)$$ My work as follows, please let me know if this is correct and where to go from here. I have hit a roadblock of sorts. $$y^{\prime\prime}+9y=9\sec^2(3t)$$...
29. Matrix Inversion for Variation of Parameters

I am working on the following problem: Can someone please show or explain the steps to invert the phi matrix? I've given it a few tries, but I can't reach what the book has for the answer. Please help! Thanks
30. MHB Method of Reduction of Order and Variation of Parameters

Hi everyone, :) One of my friends gave me the following question. I am posting the question and the answer here so that he could check his work. Question: This question concerns the differential equation, $x\frac{d^{2}y}{dx^2}-(x+1)\frac{dy}{dx}+y=x^2$ and the associated homogeneous...
31. Variation of parameters applied to an ODE

The ODE to solve via variation of parameters is ##(1-x)y''+xy'-y=(1-x)^2##. Knowing that ##e^x## and ##x## are solutions to the homogeneous ODE. Now if I call ##y_1=x## and ##y_2=e^x##, the Wronskian is ##W(y_1,y_2)=e^{x}(x-1)##. According to...
32. Variation of parameters for a second order ODE

Homework Statement I must solve ##y''+2y'+2y=e^{-t}\sin t##. I know variation of parameters might not be the fastest/better way to solve this problem but I wanted to practice it as I never, ever, could solve a DE with it. (Still can't with this one). Though the method is supposed to work...
33. Solve by using variation of parameters

x²y"(x)-3xy'(x)+3y(x)=2(x^4)(e^x) =>y"(x)-(3/x)y'(x)+(3/x²)y(x)=2x²e^x i don't know how to approach this problem because the coefficients are not constant and i am used to being given y1 and y2 HELP!
34. Couldn't understand the proof for Method of variation of parameters

Here's the proof that I read for method of variation of parameters- https://www.physicsforums.com/attachment.php?attachmentid=52267&stc=1&d=1351081780 What I couldn't understand is that how could one simply assume that u'1y1+u2'y2=0 and u'1y'1+u2'y'2=g(x) I just don't understand...
35. Question on assumptions made during variation of parameters

I was recently trying to prove the variation of parameters formula for an nth degree equation, and I have come up with a question about the assumptions made during the derivation. During the derivation we assume that: u1'y1(k) + u2'y2(k) + . . . + un'yn(k) = 0 for k < n-1. It leads to the...
36. Method of Variation of Parameters

Homework Statement y''-2y'+y = \frac{e^x}{1+x^2} Homework Equations u_1 = -\int \frac{y_{2}g(x)}{W}dx u_2 = \int \frac{y_{1}g(x)}{W}dx g(x) = \frac{e^x}{1+x^2} W is the wronskian of y1 and y2. The Attempt at a Solution The characteristic equation for the homogenous solution...
37. Variation of Parameters herupu

Homework Statement y'' + y' = 4t Homework Equations Use Variation of parameters! The Attempt at a Solution So I get homo of: c1 + c2 e^-(t) From there I get a Wronskian of -e^(-t) Then I get variations 2t^2 and -4e^t(t-1) Then get the answer of 2t^2 + 4t - 4 Btu...
38. Solve With Variation of Parameters

Homework Statement Find the particular solution to t^2 y'' - t(t + 2)y' + (t+2)y = 2t^3 given that y1 = t and y2 = tet are solutions. Also, require that t > 0 The Attempt at a Solution Rewrite the original equation as y'' - ((t + 2)/t)y' + ((t+2)/t^2)y = 2t So first I calculate the...
39. Solve With Variation of Parameters

Homework Statement Find the particular solution to t^2 y'' - t(t + 2)y' + (t+2)y = 2t^3 given that y1 = t and y2 = tet are solutions. Also, require that t > 0 The Attempt at a Solution Rewrite the original equation as y'' - ((t + 2)/t)y' + ((t+2)/t^2)y = 2t So first I calculate the...
40. Solve Differential Equation Using Variation of Parameters

Homework Statement Solve y''+25y=10sec(5t) Homework Equations NA The Attempt at a Solution I believe I have the correct answer for yp which is: 2/5log(cos(5t))cos(5t)+2tsin(5t) When I plug this into the Webwork field, it says it is incorrect. I checked my answer against...
41. Stuck on Variation of Parameters: Help with a Calculus Problem

Hey ya'll! This is the equation under discussion: y'' - 2y' - 3y = x + 2 I'm asked to use the method of variation of parameters to determine a solution for this differential equation, but I reach a point where my the equations just look too ridiculous to continue. The point I have in...
42. Variation of parameters. unsure why my solution differs from professor's

Homework Statement what is general solution of 2y'' - 3y' + y = ((t^2) + 1)e^tHomework Equations my particular solution is: (e^t) ((2/3)(t^3) + 6t -4)) prof particular solution is: ((1/3)(t^3)(e^t)) - 2(t^3)(e^t) + 9(te^t) The Attempt at a Solution here is how i solved , i hope this is ok to...
43. Solve 3rd order ode using variation of parameters

Homework Statement Solve using variation of parameters y''' - 2y'' - y' + 2y = exp(4t) Homework Equations Solve using variation of parameters The Attempt at a Solution I got the homogenous solutions to be 1, -1, and 2. So, y = Aexp(t) + Bexp(-t) + Cexp(2t) + g(t) I got...
44. Variation of parameters to obtain PS of 2nd Order non-hom equation

The question I'm trying to solve is: y" - 6y' + 9y = \frac{exp(3x)}{(1+x)} I formulated the Gen solution which are: y1(x) = exp(3x) and y2(x) = xexp(3x) I've then calculated the wronskian to get: exp(6x) I then went onto to use the variation of parameters formula, which is where...
45. Variation of parameters

I just realized you can use variation of parameters (VOP) to solve for homogeneous 2nd order equations. I see it takes much longer to do so. But I was wondering why, if you use VOP, the u and v functions are 0. Is this because the coefficients of the homogeneous equation are constant, or...
46. 4th Order Variation of Parameters

Find the complementary solution of y^\left(4\right) + 2y'' + y = sint Homogeneous Form would be y^\left(4\right) + 2y'' + y = 0 r^4 + 2r^2 + r = 0 \rightarrow r(r^3 + 2r + 1) = 0 This is where I'm stuck. Once I find y_c(t) I should be able to finish the problem, but I'm having trouble at this...
47. Solving DE Using Variation of Parameters & Given Solution

Homework Statement I must solve (1-x)y''+xy'-y=(1-x)^2 knowing that y=x is a solution if the right hand side is 0. I must use this fact in order to obtain the general solution to the DE Homework Equations Variation of parameters? The Attempt at a Solution I'm looking at...
48. Solving a DE: Variation of Parameters & Integration Issues

I've picked up a bit more since my last problem. I need to solve the following DE: x^{2}\frac{dy}{dx}+x(x+2)y=e^{x} I decided to use variation of parameters, so I re-arranged it like so: \frac{dy}{dx}=\frac{e^{x}}{x^{2}}-(1+\frac{2}{x})y Then solved the homogenous DE...
49. Solving An Initial Value DE Using Variation of Parameters

I need to find a solution to the following problem: (x^{2}-1)\frac{dy}{dx}+2y=(x+1)^{2} y(0)=0 I decided to try using variation of parameters. My teacher was unable to show any examples, and I'm having issues understanding the textbook. From what I see I need to get it onto this form...
50. Variation of parameters question

Homework Statement Using the variation of parameters method, find the general solution of x^{2}y" - 4xy' + 6y= x^{4}sin(x) Homework Equations y_{P}=v_{1}(x)y_{1}(x) + v_{2}(x)y_{2}(x) v_{1}(x)'y_{1}(x) + v_{2}'(x)y_{2}(x)=0 v_{1}(x)'y_{1}(x)' + v_{2}'(x)y_{2}(x)'=x^{4}sin(x)...