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Multiple Choice
Suppose a continuous function is defined on the closed interval , and is a critical point in . Which of the following is true about the curve at the point ?
A
is always a global minimum if .
B
is always a global maximum if .
C
could be a location of a global maximum or minimum, but endpoints or must also be checked.
D
cannot be a global extremum unless .
Verified step by step guidance
1
Step 1: Recall the definition of a critical point. A critical point occurs where the derivative of the function, f'(x), is either zero or undefined. In this case, x = c is given as a critical point, so f'(c) = 0 or f'(c) is undefined.
Step 2: Understand the role of the second derivative, f''(x), in determining the nature of the critical point. If f''(c) > 0, the function has a local minimum at x = c. If f''(c) < 0, the function has a local maximum at x = c. If f''(c) = 0, the second derivative test is inconclusive, and further analysis is needed.
Step 3: Recognize that global extrema (global maximum or minimum) on a closed interval [a, b] must be determined by evaluating the function at critical points within the interval and at the endpoints x = a and x = b. This is because the global extremum could occur at the endpoints, not just at critical points.
Step 4: Analyze the given statements. The first statement, 'x = c is always a global minimum if f'(c) = 0,' is incorrect because f'(c) = 0 only indicates a critical point, not necessarily a global minimum. The second statement, 'x = c is always a global maximum if f''(c) < 0,' is also incorrect because f''(c) < 0 indicates a local maximum, not necessarily a global maximum.
Step 5: Conclude that the correct answer is: 'x = c could be a location of a global maximum or minimum, but endpoints x = a or x = b must also be checked.' This is because global extrema on a closed interval require evaluating the function at all critical points and endpoints.