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> Find a unit vector u in which the directional derivative of $f(x,y)=\ln(1-x^2-y^2)$ at ($\frac{1}{2} ,\frac{1}{2}$) is zero
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$f\_x=\frac{-2x}{1-x^2-y^2}$ and $f\_y=\frac{-2y}{1-x^2-y^2}$ 

Giving us the gradient at $\frac{1}{2} ,\frac{1}{2}$ to be $-2,-2$

How do I now find the unit vector?