Answer
(a) Positive
(b) Negative
Work Step by Step
(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 $f$ 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.
