The Definition of the Derivative

The derivative at a point

The derivative of a function \(f(x)\) at a point \( (a, f(a))\) is written as \(f'(a)\) and is defined as a limit.

\[f'(a)=\lim_{h\rightarrow 0}\frac{f(a+h)-f(a)}{h}\]

The value of

\[\frac{f(a+h)-f(a)}{h}\]

is the slope of the line through the points \( (a, f(a))\) and \( (a+h, f(a+h))\), the so called secant line.

Note that \(\Delta x = a+h-a=h\) and \(\Delta y = f(a+h)-f(a)\). The limit of the secant lines as \(h\) tends to zero is the tangent line. The derivative is the slope of the tangent line to the graph at the point where \(x=a\). Finding the derivative is called differentiating.

Move the red dot! Change h!

Where on the graph is the derivative: positive, negative, zero? Does it matter if you approach the point from right or left, if h is positive or negative?

Taking the absolute value of the function above, you get the graph below.

Move the red dot! Change h!

In the graph above: are there any points that makes defining the derivative difficult?

The derivative as a function

You can extend the definition of the derivative at a point to a definition concerning all points (all points where the derivative is defined, i.e. where the limit exists); if doing so you get a new function \(f'(x)\) defined like this:

\[f'(x)=\lim_{h\rightarrow 0}\frac{f(x+h)-f(x)}{h} \]

Finding an algebraic formula for the derivative of a function by using the definition above, is sometimes called differentiating from first principle.

By using a computer you can find numerical approximations of the derivative at all points of the graph.

The line shown in the construction below is the tangent to the graph at the point A. The slope of the tangent line is the y-value of the point P. Drag the point A to see the trace of the point P, this trace is the graph of the derivative function.

Drag the red dot to see the graph of f'(x)!

Try to change the constant term in the definition of the function to move the graph two units upward, i.e.

\[f(x)=-0.5x^3+x^2+2x+1\]

Note that the point P still has the same trace. There are infinitely many functions giving rise to the same derivative. These functions differ by a constant but their graphs have the "same shape".

Another way of writing it

If you instead use \(y\) to denote a function of a variable \(x\), \(y=f(x)\), then you use the notation \(\frac{dy}{dx}\) to denote the derivative. The notation stems from the definition

\[\frac{dy}{dx}=\lim_{\Delta x \rightarrow 0}\frac{\Delta y}{\Delta x}\]

further info:

Wikipedia: Leibniz's notation

by Malin Christersson under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 Sweden License

www.malinc.se