3.5.3 Derivatives of Unknown FunctionsDifferentiating a known function gives an explicit result.
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Differentiating an unknown function f gives a result in terms of f'.
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Mathematica applies the chain rule for differentiation, and leaves the result in terms of f'.
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Differentiating again gives a result in terms of f, f' and f''.
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When a function has more than one argument, superscripts are used to indicate how many times each argument is being differentiated.
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This represents  . Mathematica assumes that the order in which derivatives are taken with respect to different variables is irrelevant.
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You can find the value of the derivative when  by replacing x with 0.
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Output forms for derivatives of unknown functions.
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