## Problem on finding a directional derivative

Let $f(x,y) = x^2/y$. Compute the directional derivative of $f$ at $(1,2)$ in the direction of $\mathbf{i} + 3\ \mathbf{j}$.
• ## Solution

Recall that
To apply this formula, we find $\bfv$ and compute $\nabla f(x,y)$ at $(1,2)$.
Applying the formula for directional derivative, we get