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The notation $\frac{d^2 f}{dx^2}$ is a standard shorthand for the expanded differential form $\frac{d(\frac{df}{dx})}{dx}$. The expanded form literally means taking the differential of the first derivative and dividing by the differential of $x$.

Conditions: The function $f$ is twice differentiable.; Leibniz notation conventions are being used.

The notation $\frac{d^2 f}{dx^2}$ is a standard shorthand for the expanded differential form $\frac{d(\frac{df}{dx})}{dx}$. The expanded form literally means taking the differential of the first derivative and dividing by the differential of $x$.

Conditions: The function $f$ is twice differentiable.; Leibniz notation conventions are being used.