approaches a [[limit]]ing value that we call the [[derivative]] of the function f(x) at ''x=a''.
approaches a [[limit]]ing value that we call the [[derivative]] of the function f(x) at ''x=a''.
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There is also the more rigorous <math>\epsilon-\delta</math> definition: a function f is said to be differntiable at point ''a'' if ∀<math>\epsilon>0</math> ∃<math>\delta>0</math> such that if