While calculating a surface integral, how do you know which variable to use as $u$ and $v$ in the calculation?
For example in Cartesian coordinates you have $x$, $y$ and $z$.
In cylindrical $\theta$, $r$ and $z$.
And in polar $\theta$, $\rho$ and $\phi$.
I’m confused as to which two should you choose as $u$ and $v$ to work out the question as I end up with different answers.