Differential Differential is the linear principal part of the amount of change of a variable in a process of change. If the function y=f(x) has a derivative f'(x) at the point x, the change of y due to the change of x △x is △y=f(x+△x)-f(x) =f'(x)·△x+o(△x), where o(△x) tends to 0 with △x. So the main part of the linear form of Δy is dy=f'(x) Δx is the differential of y.


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