2nd order PDE using integration by parts

[perishingtardi]

Homework Statement

Find the general solution of the equation
(\zeta - \eta)^2 \frac{\partial^2 u(\zeta,\eta)}{\partial\zeta \, \partial\eta}=0,
where $\zeta$ and $\eta$ are independent variables.

Homework Equations

The Attempt at a Solution

I set $X = \partial u/\partial\eta$ so that (\zeta - \eta)^2 \frac{\partial X}{\partial\zeta}=0. Then \int (\zeta - \eta)^2 \frac{\partial X}{\partial\zeta} \, d\zeta=f(\eta). I used integration by parts to obtain
(\zeta - \eta)^2X - 2\int \zeta X \, d\zeta + 2\eta \int X\, d\zeta = f(\eta), but I'm not sure if this is the correct method, or how to proceed.

 

[dirk_mec1]

Hint: what is
\frac{ \partial \zeta} { \partial\eta}

 

[perishingtardi]

Hint: what is
\frac{ \partial \zeta} { \partial\eta}

its zero?? how does that help though?