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## Homework Statement

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,

https://www.physicsforums.com/attachment.php?attachmentid=54937&stc=1&d=1358917223

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?

Thank you