Mean Value Theorems
For f(x) = x^3 - 3x on the closed interval [-\sqrt{3}, \sqrt{3}], how many values of c satisfy Rolle's theorem?
Select the correct option:
Solution
2
Rolle's theorem applies to a function continuous on a closed interval, differentiable on the open interval, and taking equal values at the endpoints, guaranteeing at least one interior point where the derivative vanishes. The polynomial f is continuous and differentiable everywhere, and f(-\sqrt{3}) = (-\sqrt{3})^3 - 3(-\sqrt{3}) = -3\sqrt{3} + 3\sqrt{3} = 0 = f(\sqrt{3}), so all hypotheses hold. Differentiate: f'(x) = 3x^2 - 3, set to zero to get x^2 = 1, so x = \pm 1. Both x = 1 and x = -1 lie strictly inside (-\sqrt{3}, \sqrt{3}) since 1 < \sqrt{3} \approx 1.732, and both satisfy f'(c) = 0, so there are exactly two values of c that fulfil Rolle's conclusion. Option 0 is impossible because the hypotheses hold and f' does vanish inside. Option 1 under-counts by reporting only Rolle's minimum guarantee of one point rather than the actual number of interior stationary points. Option 3 over-counts beyond the two roots of the quadratic f'. The governing pattern is Rolle's theorem on a symmetric cubic, whose derivative is a quadratic with two real roots. Plausibility check: f' = 3x^2 - 3 is an upward parabola crossing zero at x = -1 and x = 1, both well within the symmetric window (-\sqrt{3}, \sqrt{3}), giving exactly two Rolle points.
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About This Question
- Subject
- mathematics
- Chapter
- limit, continuity and differentiability
- Topic
- mean value theorems
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
2
Rolle's theorem applies to a function continuous on a closed interval, differentiable on the open interval, and taking equal values at the endpoints, guaranteeing at least one interior point where the derivative vanishes. The polynomial f is continuous and differentiable everywhere, and f(-\sqrt{3}) = (-\sqrt{3})^3 - 3(-\sqrt{3}) = -3\sqrt{3} + 3\sqrt{3} = 0 = f(\sqrt{3}), so all hypotheses hold. Differentiate: f'(x) = 3x^2 - 3, set to zero to get x^2 = 1, so x = \pm 1. Both x = 1 and x = -1 lie strictly inside (-\sqrt{3}, \sqrt{3}) since 1 < \sqrt{3} \approx 1.732, and both satisfy f'(c) = 0, so there are exactly two values of c that fulfil Rolle's conclusion. Option 0 is impossible because the hypotheses hold and f' does vanish inside. Option 1 under-counts by reporting only Rolle's minimum guarantee of one point rather than the actual number of interior stationary points. Option 3 over-counts beyond the two roots of the quadratic f'. The governing pattern is Rolle's theorem on a symmetric cubic, whose derivative is a quadratic with two real roots. Plausibility check: f' = 3x^2 - 3 is an upward parabola crossing zero at x = -1 and x = 1, both well within the symmetric window (-\sqrt{3}, \sqrt{3}), giving exactly two Rolle points.
This medium difficulty mathematics question is from the chapter limit, continuity and differentiability, covering the topic of mean value theorems. It appeared in the 2025 exam.
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