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Why do two upward-curving graphs at the same point x=4x=4 have different positive second derivatives, 10 versus 0.4?

Although both graphs curve upward (indicating a positive second derivative), the narrow parabola has a much more rapid increase in slope around x=4x=4 compared to the wider parabola. Since the second derivative measures the rate of change of the slope, a faster change in slope results in a larger numerical value.

Conditions

  • Both graphs are evaluated at the same input x=4x=4.
  • Both graphs are curving upward near x=4x=4.

Reasoning, step by step

  1. Compare the shape of the two upward-opening parabolas at x=4x=4.
  2. Observe that the narrower parabola's tangent slope changes very quickly over a small horizontal distance.
  3. Observe that the wider parabola's tangent slope changes slowly.
  4. Link the speed of slope change directly to the magnitude of the second derivative: rapid change yields 10, slow change yields 0.4.

Example

At x=4x=4, the screen shows d2fdx2(4)=10\frac{d^2 f}{dx^2}(4)=10 for the narrow parabola and d2fdx2(4)=0.4\frac{d^2 f}{dx^2}(4)=0.4 for the wide parabola, illustrating that curvature sharpness affects the second derivative value.

Common misconceptions

  • Believing that any upward curve must have the same second derivative value regardless of its 'sharpness'.
  • Confusing the second derivative with the function value or the first derivative alone.

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