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?

Hold $b$ and $c$ constant to find the derivative with respect to $a$. Thus
\begin{equation}
\frac{\partial y}{\partial a} = \frac{0.99821}{c-b}
\end{equation}
Similarly
\begin{align}
\frac{\partial y}{\partial b} &= \frac{\partial}{\partial b}\left(\frac{0.99821(a-b)}{c-b}\right)\\
&= \frac{1}{(c-b)^{2}}\left(-0.99821(c-b)-(-0.99821(a-b)) \right) \\
&= \frac{0.99821}{(c-b)^{2}}\left(a-c\right)
\end{align}
Where the second line above is found by using the quotient rule. The derivative with respect to $c$ is found similarly. 
