- Thread starter
- #1

\[

(1 - c^2)U_{\eta\eta} = \sin(u).

\]

My question is the chain rule on the differential.

\[

U_{\eta} = \frac{\partial u}{\partial x} \frac{\partial x}{\partial\eta} + \frac{\partial u}{\partial t} \frac{\partial t}{\partial\eta}

\]

but this doesn't seem to work out correctly.