## Calculus: Early Transcendentals 8th Edition

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