derivatives, and the gradient

\\filer\calclab

when you hit enter, you can then choose MA1024 and then choose the worksheet

Pardiff_grad_start_B11.mw

Remember to immediately save it in your own home directory. Once you've copied and saved the worksheet, read through the background on the internet and the background of the worksheet before starting the exercises.

The Maple commands for computing partial derivatives are `D`
and `diff`. The **Getting Started** worksheet has examples
of how to use these commands to compute partial derivatives.

A crucial first step in solving such problems is being able to find and classify local extreme values of a function. What we mean by a function having a local extreme value at a point is that for values of near , for a local maximum and for a local minimum.

In single-variable calculus, we found that we could locate candidates for local extreme values by finding points where the first derivative vanishes. For functions of two variables, the condition is that both first order partial derivatives must vanish at a local extreme value candidate point. Such a point is called a stationary point. It is also one of the three types of points called critical points. Note carefully that the condition does not say that a point where the partial derivatives vanish must be a local extreme point. Rather, it says that stationary points are candidates for local extrema. Just as was the case for functions of a single variable, there can be stationary points that are not extrema. For example, the saddle surface has a stationary point at the origin, but it is not a local extremum.

Finding and classifying the stationary points of a function requires several steps. First, the partial derivatives must be computed. Then the stationary points must be solved for by solving where both partial derivatives equal zero simultaneously.

Next, one must check for the presence of singular points, which might also be local extreme values. Finally, each critical point must be classified as a local maximum, local minimum, or a saddle point using the second-partials test:

If and then is a local minimum.

If and then is a local maximum.

If then is a saddle point.

If then no conclusion can be made. The examples in the Getting Started worksheet are intended to help you learn how to use Maple to simplify these tasks.

to compute the directional derivative. However, the following computation, based on the definition, is often simpler to use.

One way to think about this that can be helpful in understanding directional derivatives is to realize that is a straight line in the plane. The plane perpendicular to the plane that contains this straight line intersects the surface in a curve whose coordinate is . The derivative of at is the rate of change of at the point moving in the direction .

Maple doesn't have a simple command for computing directional
derivatives. There is a command in the `tensor` package that
can be used, but it is a little confusing unless you know something
about tensors. Fortunately, the method described above and the method
using the gradient described below are both easy to implement in
Maple. Examples are given in the `Getting Started` worksheet.

As described in the text, the gradient has several important properties, including the following.

- The gradient can be used to compute the directional derivative
as follows.

- The gradient points in the direction of maximum increase of the value of at .
- The gradient is perpendicular to the level curve of that passes through the point .
- The gradient can be easily generalized to apply to functions of three or more variables.

Maple has a fairly simple command `grad` in the `linalg`
package (which we used for curve computations). Examples of computing
gradients, using the gradient to compute directional derivatives, and
plotting the gradient field are all in the `Getting Started`
worksheet.

- Using the method from the
`Getting Started`worksheet, compute the directional derivative of at the point in each of the directions below. Explain your results in terms of being positive, negative or zero and what that tells about the surface at that point in the given direction. - Repeat the above exercise using the points and in the same directions. What do your results suggest about the surface at these points? What is the difference between a zero answer in this exercise compared to the zero answer from exercise 1?
- Find and classify the stationary points for using the second derivative test.
- Using the method from the
`Getting Started`worksheet, plot the gradient field and the contours of on the same plot over the intervals and . Use 30 contours, a grid and`fieldstrength=fixed`for the gradient plot. Describe how this plot supports your answer in the previous exercise using both the gradient field and the countour plot in your explanation.

2011-11-17