Max and Min's |

Can you find the local maximum and local minimum in the graph above? Yes, of course. Now look at the same places and think about what the slope is at those two locations. Correct, the slope is zero at those locations. Is the slope equal to zero anywhere else on the graph? The answer is no. So if you want to find maximums or minimums a good way to get started is to find out where the slope of the function is equal to zero. When you don't have a graph to look at the best way to find where the slope is zero is to set the derivative equal to zero.

Critical Points

Let's go through an example. Given **f(x) = x ^{3}-6x^{2}+9x+15** , find any and all local maximums and minimums.

__ Step 1. f '(x) = 0__, Set derivative equal to zero and solve for "x" to find critical points. Critical points are where the slope of the function is zero or undefined.

f(x) = x^{3}-6x^{2}+9x+15

f '(x) = 3x^{2}-12x+9

3x^{2}-12x+9 = 0

3(x^{2}-4x+3) = 0

3(x-1)(x-3) = 0

x=1, or x=3.

Ok, now we have our critical points but which one is the minimum and which one is the maximum? Don't try to guess. We don't have enough information yet.

We need to find out more about what is happening near our critical points. Ok your right, we need to find out what is happening on either side of our critical points.

Shot Gun Method

__ Step 2 Option 1.__ Pick numbers on either side of the critical points to "see what's happening". But what exactly are we looking for? Well let's say we use

Can you see why this is sometimes called the shotgun method? Let's connect the "buckshot" and get a better idea of the shape of the function.

We could do something similar for **x=3** but let's not.

First Derivative Test

__ Step 2 Option 2.__ Let's try another technique. How about we use just the numbers on either side and see what the derivative says about the function at those locations. (This boils down to only have to test two numbers instead of testing three.) If we use

The function must follow the path of the arrows and we can conclude that the function must have the following shape and there is a __local maximum__ at **x=1**.

Thus we can see from above that is the function is increasing before **x=1** and decreasing after **x=1**, then **x=1** has to be a __local maximum__. Let's now look at the other critical point, **x=3**. To finish our analysts we only need to find out about the slope of the function after **x=3**. Let's test **x=4** in the derivative** f '(x) = 3x ^{2}-12x+9.** Since

With this extra information about the slopes, we get an even more complete picture of the basic shape of the function. We can see that there is also a __local minimum __at **x=3**.

Using the first derivative to test numbers on either side of the critical points to see if the function is increasing or decreasing is commonly called the First Derivative Test. It works pretty well and is almost guaranteed to be hassle free provided that you can handle derivatives. But if you are comfortable using derivatives then you might as well learn about the Second Derivative Test. It is even fast then the First Derivative Test.

Second Derivative Test

__ Step 2 Option 3.__ Let's redo the above example continuing from where we just found the critical points but don't know anything else about the function.

Given **f(x) = x ^{3}-6x^{2}+9x+15**, the derivative is still

You can basically look at the above picture and see where the local maximum and local minimum is. Since the function is concave down at **x=1** and has a critical point at **x=1** (zero slope) then the function has a local maximum at **x=1**. Since the function is concave up at **x=3** and has a critical point at **x=3** (zero slope) then the function has a local minimum at **x=3**. Below is the same information with a possible shape of f(x).

In summary after finding the critical points you can use any one of the three methods above to determine if they are a local maximum or a local minimum (or possible neither?). For most functions it does not matter which method you use because they will all work. When given the freedom to choose you might prefer the second derivative test because it is a little faster than the other two. Sometimes the second derivative test does not work as you had plan and you will need to go back and use one of the other two methods (first derivative test next easiest method).

To learn more about what to do if the second derivative test doesn't work as you hoped or how a critical point might __not__ be a local maximum or a local minimum you can continue with the next lesson on Inflection Points.

Note: the section on inflection points is usually covered in Math 34B but some teachers will cover it in Math 34A.