## Calculus 8th Edition

(a) Since, the graph of $f$ increases for $z$, if we start at $(1, 2)$ and move in positive x- direction, this represents the positive values of $f$. Thus,$f_{x} (1,2)$ is positive. (b) Since, the graph of $f$ decrease, if we start at $(1, 2)$ and move in positive y- direction, this represents the negative values of $f$. Thus,$f_{y} (1,2)$is negative.