- #1
tom_rylex
- 13
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Homework Statement
I am looking at the derivation of the D'alembert equation, and I'm having trouble with understanding where the limits of integration come in.
Homework Equations
Given the 1-d wave equation:
[tex] u_{tt} = c^2u_{xx} [/tex], with the general solution [tex] u(x,t)= \theta(x-ct) + \psi(x+ct) [/tex] and the initial conditions
[tex] u(x,0)=f(x) [/tex], [tex] u_t(x,0)=g(x) [/tex]
Show that the solution is
[tex] u(x,t)=\frac{1}{2} \left[ f(x+ct) + f(x-ct) +\frac{1}{c}\int_{x-ct}^{x+ct} g(y)dy \right] [/tex]
The Attempt at a Solution
If I take the second of the initial conditions, I get
[tex] -c\theta'(x)+c\phi'(x)=g(x) [/tex]
[tex] -\theta(x)+\phi(x)=\frac{1}{c}\int g(x) [/tex],
I guess I just don't understand where the limits of integration come from to yield
[tex] \frac{1}{c} \int_{-\infty}^x g(y) dy [/tex]
on the right hand side.