 Then there is a unique value of "k" such that, as "h" approaches 0, the difference quotient gets closer and closer to "k", and the distance between them becomes negligible compared with the size of "h", if "h" is small enough.This leads to the definition of the slope of the tangent line to the graph as the limit of the difference quotients for the function "f".This limit is the derivative of the function "f" at "x" = "a", denoted "f" ′("a").