Having trouble with partial derivatives I am having trouble calculating partial derivatives of a simple function. 
The function is:
$$ y(a,b,c)=\frac {0.99821*(a-b)}{c-b} $$
And I need to calculate $ \frac {\partial y}{\partial a} $, $\frac {\partial y}{\partial b}$ and $\frac {\partial y}{\partial c}$ .
If I am not wrong, to calculate $\frac {\partial y}{\partial a}$, I need to consider $b$ and $c$ as constants. However, derivatives of constants are always 0, thing that will make the denominator of the initial function equal with 0.
Where am I mistaking?

and last but not least $$\frac{\partial y}{\partial c}=- 0.99821\,{\frac {a-b}{ \left( c-b \right) ^{2}}}$$
