Help understanding a partial derivative $\frac{\partial\pi}{\partial C}$ This is one of my first problems involving partial derivatives and I don't understand how the partial works out to be $\frac{1}{d}$ given that $\pi=\frac{C}{d}$
I have to find:
$\frac{\partial\pi}{\partial C}$ and $\frac{\partial\pi}{\partial d}$
And I don't see how they'd turn out to be anything other than $0$ since, as I understand it, $C$ and $d$ are treated as constants depending on which derivative I'm taking.  

d(pi)/dC = 1/d because C/d = C*(1/d) and d is fixed. Likewise, d(pi)/dd = -C/d^2 because C is fixed.
