State whether each statement is true of false. Justify your response for each.

1. If f(x,y) = v1-x2 - y', then Daf(0,0) =0 for any unit vector u. 2. If Daf(x, y) exists, then Daf(r, y) = -D_af(x, y). 3. If Daf (co, yo) = c for any unit vector u, then c = 0