stuck on logarithm of derivative of sum $\frac{\partial\mathrm{log}(a+b)}{\partial a}$ I need to evaluate an expression similar to the following:
$\frac{\partial\mathrm{log}(a+b)}{\partial a}$
At this point I don't know how to proceed. $b$ is a constant so there should be some way to eliminate it.
How would you proceed in this case?
Actually, the original expression is much more complicated, and related to multiclass logistic regression, but I wanted to spare you tedious details.

Clearly, the answer is
$\frac{\partial log(a+b)}{\partial a} = \frac{1}{a+b}$
As you said, b is a constant but in this case it is also in an argument of a derivated function and thus cannot be eliminated using derivatives.
