1. The picture below shows the graph of a function f (x, y). Z Suppose the red point has coordinates (a, b, f(a, b) ) and the blue point has coordinates (c, d, f (c, d)). Let ). Determine whether each of the following partial derivatives is positive, negative, or zero: a. fx(a, b) b. fy (a, b ) c. Da(a, b) d. fx(c, d) e. fy (c, d) f. Da(c, d)