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Mean Value Theorems

Mediummathematics

For f(x) = x^3 - 3x on the closed interval [-\sqrt{3}, \sqrt{3}], how many values of c satisfy Rolle's theorem?

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About This Question

Subject
mathematics
Chapter
limit, continuity and differentiability
Topic
mean value theorems
Difficulty
Medium
Year
2025
Tags
advanced-calculus-drillrolle-theoremmean-value-theoremstationary-pointspolynomial-roots

Solution

Correct Answer:

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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