a is a constant. My brain is fried from two days of aerodynamics, i can`t get this one.
Hello flawless$\displaystyle z = \tan^{-1}u$
$\displaystyle \Rightarrow \tan z = u$
$\displaystyle \Rightarrow \frac{du}{dz} = \sec^2 z = 1 + \tan^2 z = 1 + u^2$
$\displaystyle \frac{dz}{du}= \frac{1}{1+u^2}$
For the partial derivative, assume $\displaystyle y$ is constant (as well as $\displaystyle a$) and differentiate with respect to $\displaystyle x$. Can you complete it now?
Grandad