Please have a look at the picture attach, which shows the proof of the D'alembert's solution to the wave equation. If you can't open the open,
click onto this link:http://www-solar.mcs.st-and.ac.uk/~alan/MT2003/PDE/node12.html
The part I have problem is the taking the second paritial derivative of u with respect to x.
Ux=du/dz+du/dn, does this mean that Ux is a function of z and n, that is, Ux(z,n), and z(x,t) and n(x,t)?
so later when I taken the second derivative, then I apply chain rule?