試題詳解

3. Let f(x, y) = x³ - 12xy + 8y³. Which of the following must be true?
(A) f(x, y) has three critical points
(B) (2,1) is a saddle point of f(x, y)
(C) At the point P(1, 1), f(x, y) increases fastest in the direction (-3/5,4/5)
(D) None of the above