### Applications of differentiation: Analysis of functions

### Local minima and maxima

Let's have another look at an exercise from week 4.

The function rule of the tangent line has the form #l(x)=a\cdot x+b#. S o need to find the values for #a# and #b#.

The slope #a# of the tangent has to be #a=f'(4)#. The derivative of #f# is #f'(x)=8-2\cdot x#. This means that #a=f'(4)=0#.

The formula of the tangent therefore looks like this: #l(x)=0 \cdot x +b#. You now only have to calculate #b#. This is simple. The tangent goes through #\rv{4,20}#. Hence, #b=20-0 \cdot 4 = 20#. We conclude that the function rule for the tangent is #l(x)=20#.

You can see that the tangent line is a horizontal line. If a function changes from increasing to decreasing (or from decreasing to increasing), then its derivative changes signs, so the derivative at the transition point will be #0#.

We have seen that as a function decreases, its derivative is negative and that as a function increases its derivative is positive. We can also reverse this.

Stationary points

Let #f# be a differentiable function. A point #x# with #f'(x)=0# is called a **stationary** point of said #f#.

Stationary points say something about how a function behaves, because a function can only change from increasing to decreasing, and vice versa, in a stationary point.

First, we will now take a look at the definition of a local maximum and minimum before we look at how we can use the stationary point to find these extremes.

We can say that #f# has a **local maximum** in #p# if there is an open interval #\ivoo{c}{d}# around #p# with #f(x)\le f(p)# for all #x\in\ivoo{c}{d}\cap I#.

We can say that #f# has a **local minimum** in #p# if there is an open interval #\ivoo{c}{d}# around #p# with #f(x)\ge f(p)# for all #x\in\ivoo{c}{d}\cap I#.

All local minima and maxima together we call

**extreme points.**

Points at the edge of #I# can also be local maximums. If a local maximum is not on the edge of #I#, then the open interval #\ivoo{c}{d}# can be chosen completely in #I#.

Local extrema, stationary points

If #I# is an open interval, #f# is differentiable in #p#, and #p# is a local maximum or minimum of #f#, then #f'(p)=0#.

So, a local maximum or minimum always is a stationary point.

Please note that as you can see in the first example below, a stationary point is not always a local maximum or minimum.

After all, we need to provide a differentiable function #f# with the following properties:

- #f'(0)=0# (this means that #p=0# is a stationary point of #f#)
- #0# is neither a local minimum nor a local maximum of #f#

The value of #f# at #0# is #0#. Since #x^3\lt0# if #x\lt0# and #x^3\gt 0# if #x\gt0#, each interval around #0# contains points at which the value of #f# is greater than #f(0)# and points at which the value of #f# is smaller than #f(0)#. In particular, #p=0# is neither a local minimum nor a local maximum of #f#.

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