# Thread: Partial differentiation with a log function

1. ## Partial differentiation with a log function

How do I do this particular question?
f (x,y) = ln (x + y)

2. What do you actually want to compute? Is it $\displaystyle \frac{\partial f}{\partial x}$ or $\displaystyle \frac{\partial f}{\partial y}$?

3. Originally Posted by Krizalid
What do you actually want to compute? Is it $\displaystyle \frac{\partial f}{\partial x}$ or $\displaystyle \frac{\partial f}{\partial y}$?
you can pick one and do it. if that's the question, great, if not, the poster can practice by doing the other one similarly.

4. I need to do both for this question, not just one. If someone could help me with it, I would greatly appreciate it.

5. Originally Posted by njr008
I need to do both for this question, not just one. If someone could help me with it, I would greatly appreciate it.
for $\displaystyle \frac {\partial f}{\partial x}$ you treat $\displaystyle x$ as a variable and $\displaystyle y$ as a constant. so $\displaystyle y$ is just like any other number now. it might as well be 2 or something.

Thus, by the chain rule:

$\displaystyle \frac {\partial f}{\partial x} = \frac 1{x + y} \cdot 1 = \frac 1{x + y}$

do a similar thing for $\displaystyle \frac {\partial f}{\partial y}$