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Introduction to Derivatives · Ages 17–18

Second Derivative, Concavity, and Inflection Points Practice

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Designed for 12th-grade calculus students. Work through the problems in order to determine second derivatives, intervals of concavity, and coordinates of inflection points.

  1. Find the second derivative of f(x) = 4x^3 - 5x^2 + 7x - 1.

  2. Find all candidate inflection points for f(x) = x^3 - 3x^2 + 4 by solving f''(x) = 0.

  3. For f(x) = x^3 - 3x^2 + 4, determine the intervals where the function is concave down and concave up.

  4. Identify the coordinates of the inflection point for f(x) = x^3 - 3x^2 + 4.

  5. Determine whether f(x) = x^4 has an inflection point at x = 0. Explain why or why not.

  6. Given f'(x) = 0 at x = 3 and f''(3) = -8, classify the point at x = 3 using the second derivative test.

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Second Derivative, Concavity, and Inflection Points Practice

Answer key

  1. Find the second derivative of f(x) = 4x^3 - 5x^2 + 7x - 1.

    f''(x) = 24x - 10

  2. Find all candidate inflection points for f(x) = x^3 - 3x^2 + 4 by solving f''(x) = 0.

    x = 1

  3. For f(x) = x^3 - 3x^2 + 4, determine the intervals where the function is concave down and concave up.

    Concave down on (-infinity, 1); concave up on (1, infinity).

  4. Identify the coordinates of the inflection point for f(x) = x^3 - 3x^2 + 4.

    (1, 2)

  5. Determine whether f(x) = x^4 has an inflection point at x = 0. Explain why or why not.

    No. While f''(0) = 0, f''(x) = 12x^2 is positive for both x < 0 and x > 0, so concavity does not change.

  6. Given f'(x) = 0 at x = 3 and f''(3) = -8, classify the point at x = 3 using the second derivative test.

    Local maximum, because f''(3) < 0 indicates the curve is concave down at that stationary point.