Calculus

For a function of two variables $f(x, y)$, a critical point $(x_0, y_0)$ is a saddle point if $D = f_{xx} f_{yy} - (f_{xy})^2$ satisfies:

Options:

• (A) $D < 0$
• (B) $D > 0$ and $f_{xx} > 0$
• (C) $D > 0$ and $f_{xx} < 0$
• (D) $D = 0$