Suppose a function z = (x, y) satisfies the criteria for the test for relative extrema at a point (a, b), and xx (a,

Suppose a function z = ƒ(x, y) satisfies the criteria for the test for relative extrema at a point (a, b), and ƒ_xx(a, b) > 0, while ƒ_yy(a, b) < 0. What does this tell you about ƒ(a, b)? Based on the sign of ƒ_xx(a, b) and ƒ_yy(a, b), why does this seem intuitively plausible?