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The slope of the function f at the point (a,f(a)) is the limit of the slope of the secant line between x = a and x = a + h as h gets closer to 0. Whew, that's a mouthful.

From the formula for the slope of this secant line, we find another formula:

"The slope of f at a" is also called the derivative of f at a and is written f'(a). This nice formula is known as the limit definition of the derivative, and we write it again here, with correct notation:

Visually, we move h closer and closer to 0 (equivalently, move a + h closer and closer to a) and watch what happens to the secant line, as in this animation.

If we could keep going until h reached 0 (equivalently, until ah reached a) we would find a line that, instead of passing through the graph twice, would hit the graph at one spot and bounce off, as shown in this picture:

The slope of the function f at the single point x = a is the slope of this line, also called the tangent line.

Two important things to remember:

  • The slope of f at a and the derivative of f at a are the same thing.
  • Since the derivative of f at a is a limit, the derivative won't always exist

Another phrase for f'(a) is the instantaneous rate of change of f with respect to x when x = a.

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