Directional Derivatives
The directional derivative of in the direction of a unit vector at the point is denoted by D_\vec{u}f(a,b) and defined as
D_\vec{u}f(a,b) = \lim_{h \to 0}\frac{f(a+hu_1, b+hu_2)-f(a,b)}{h}
In practice, we can use D_\vec{u}f(a,b) = \nabla f(a,b) \cdot \vec{u}
Remember that $\vec{u}$ must be a **unit vector**.