Hi there. I have to find the directional derivative for:

at the point

in the direction of

.

The thing is that the derivatives for this function are not defined at the point P0. So, I can't use the proyection of the gradient over u. But using the definition I've found an answer, I think its right, but I'm not pretty sure, because the function is not well defined at the point it's asked.

This is what I did:

So, the thing is if this is the right answer, or if I should say that the derivative at that point doesn't exist.