Originally Posted by

**Ulysses** Hi there. I have to find the directional derivative for: $\displaystyle \sqrt{|xy|}$ at the point $\displaystyle P_0(0,0)$ in the direction of $\displaystyle \vec{u}\left (5/13,12/13 \right )$.

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:

$\displaystyle \displaystyle\lim_{t \to{0}}{\displaystyle\frac{\sqrt{|t^2\displaystyle \frac{5}{13} \displaystyle\frac{12}{13} |} }{t}}= \displaystyle\frac{\sqrt[ ]{60}}{13}$

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