# Homework Help: Partial derivative, I'm not sure if my solution is right

1. Sep 20, 2010

### Telemachus

1. The problem statement, all variables and given/known data
Well, it looks simple, but I'm not sure If the answer I'm giving is right.

The function is:

$$f(x,y)=\sin|y|$$
And it asks for the partial derivatives, so:

$$\displaystyle\frac{\partial f}{\partial x}=0$$

And
$$\displaystyle\frac{\partial f}{\partial y}=\begin{Bmatrix}{ \cos|y|}&\mbox{ if }& y>0\\\not{\exists & \mbox{if}& y=0\\-\cos|y| & \mbox{if}& y<0\end{matrix}$$

Is this right?

2. Sep 20, 2010

### rock.freak667

When you are defining the domains like that, you do not need to rewrite the |y|. But if y=0, then f(x,y)=0 so that ∂f/∂y = 0.

3. Sep 20, 2010

"But if y=0, then f(x,y)=0 so that ∂f/∂y = 0."

No. think about $f(x) = |x|$ from ordinary calculus. it is true $f(0) = 0$ but remember f is not differentiable @ x = 0; similar situation in the case given by the OP

4. Sep 20, 2010

### Telemachus

Its continuous but not differentiable at that point.

5. Sep 20, 2010

The absolute value function is continuous at x =0: is your partial derivative continuous in y at y = 0?

6. Sep 20, 2010

### Telemachus

Yes, it is. It can be demonstrated that Lim f(x,y)->(0,0)=f(0,0). So, what you say, the solution I've given is right?

7. Sep 20, 2010