Given the graph of a function , at the point , the surface is increasing as increases and decreasing as increases. Which of the following correctly describes the signs of the partial derivatives and ?
A
,
B
,
C
,
D
,
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1
Step 1: Understand the problem. The question asks about the signs of the partial derivatives f_x(a, b) and f_y(a, b) at the point (a, b) based on the behavior of the surface. Specifically, the surface increases as x increases and decreases as y increases.
Step 2: Recall the meaning of partial derivatives. The partial derivative f_x(a, b) represents the rate of change of the function f with respect to x at the point (a, b), while keeping y constant. Similarly, f_y(a, b) represents the rate of change of f with respect to y at the point (a, b), while keeping x constant.
Step 3: Analyze the behavior of the surface. Since the surface is increasing as x increases, this implies that f_x(a, b) > 0 because the function is growing in the positive x-direction. Conversely, since the surface is decreasing as y increases, this implies that f_y(a, b) < 0 because the function is decreasing in the positive y-direction.
Step 4: Match the signs of the partial derivatives to the given options. Based on the analysis, the correct description of the signs is f_x(a, b) > 0 and f_y(a, b) < 0.
Step 5: Conclude that the correct answer is the option stating f_x(a, b) > 0 and f_y(a, b) < 0, as this matches the behavior of the surface described in the problem.