The picture below shows the graph of a function f : R² R.

Suppose the red point has coordinates (a, b, f(a, b)) and the blue point has coordinates (c, d, f(c, d)).

Also let u =

D

1

2

,

1

2

E

. Determine whether each of the following partial derivatives is positive,

negative, or 0; explain briefly.

(a) fx(a, b)

(b) fy(a, b)

(c) fx(c, d)

(d) fy(c, d)

(e) Du(a,b)

(f) Du(c,d)