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CHAPTER 6:

THE TANGENT PLANE

6.1 INTRODUCTION TO THE TANGENT PLANE

The following diagram contains the graph of the function y = x 2 and the graph of its tangent line

at the point (1, 1)

With these graphs, it can be seen that there is a small region near the point (1, 1) where the graph

of the tangent line appears identical to the graph of y = x 2 . Correspondingly, the tangent line can

frequently be used to approximate the behavior of the graph near the point to which the line is

tangent.

In three dimensions, the tangent plane plays a similar role. We have seen that it is an easily

obtainable tool. When discussing derivatives of functions of two variables. It is important to note

that we can obtain the tangent plane to a surface very easily.

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With this graph, we can see that, as with the tangent line in 2-D, there is a small region near the

point (a, b, f (a, b)) where the behavior of the tangent plane and the behavior of the surface z = f

(x, y) appear identical.

FINDING THE TANGENT PLANE

As was discussed in the section on planes, a point, the slope in the x direction and the slope in the

y direction are sufficient information to identify and obtain the formula for a plane.

Given a surface z = f (x, y) and a point (x0, y0), the tangent plane with mx = f x (x0, y0) and my = f y

(x0, y0). And the point (x0, y0, f (x0, y0)) will lie on the tangent plane.

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As we now have a point on the tangent plane and two of its slopes, we no longer need the

surface.

We can now use the two slopes and the point to obtain the tangent plane.

Hence, obtaining the formula and the geometric representation for the tangent plane is quite

straightforward and correspondingly the tangent plane is an easily obtainable tool.

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6.2 DIFFERENTIALS

We have spoken of rise in a surface as Δz as we move from one point to another. We used to

obtain approximations for partial derivatives. However, when we are using the tangent plane as

an approximation, we can consider rise as we move along the tangent plane as well. To

differentiate between whether we are moving on the surface z = f (x, y) or the tangent plane to the

surface at some given point, the following convention will be used:

When moving along a surface, z = f (x, y), the change in height will be referred to as Δz.

When moving along the tangent plane to a surface z = f (x, y) at some given point, the

change in height will be referred to as the differential dz.

Example Exercise 6.2.1: Given the surface z = .5x 2 + y

2 , find dz if Δx = 2 and Δy = 3 and the

tangent plane at the point (1, 1, 1.5) is it to be used to approximate the surface.

Solution:

1. Obtain the tangent plane to the surface:

Using the point and the two slopes, the tangent plane will be:

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As we are seeking the differential, the surface can be discarded and we can pay attention

solely to the tangent plane in order to obtain the desired change in height. The two data we

use on the tangent plane are mx = fx(1,1) = 1 and my = fy(1,1) = 2.

2. Identify the right triangle whose rise and run are associated with these slopes.

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3. Find the change in height associated with the x direction:

4. Find the change in height associated with the y direction:

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5. Find the overall change in height: Taking the change in height associated with movement

of 2 units in the x direction and 3 units in the y direction and adding them to yield the

total change in height associated with this movement yields dz = dzx + dzy = 8.

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Generalization

Given a function z = f (x, y) and a point (x,y)=(a,b), in order to find the differential dz associated

with movement in the x direction dx and movement in the y direction dy, we can follow the

following steps.

1. Obtain the tangent plane to the surface

Using the point and the two slopes, the tangent plane will be

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As, we are seeking the differential, the surface can be discarded and we can pay attention

solely to the tangent plane in order to obtain the desired change in height.

2. Identify the right triangle whose rise and run are associated with these slopes.

3. Find the change in height associated with the x direction:

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The rise in the x direction dzx = fx(a,b) dx

4. Find the change in height associated with the y direction:

The rise in the y direction dzy = fy(a,b)dy

5. Taking both documents of the change in height on the tangent plane or the differential is

dz = dzx + dzy = f x dx + f y dy

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6.3 DIRECTIONAL DERIVATES

DEFINITION

D f (x0, y0) is defined as the slope of the tangent line to z = f (x, y) at the point (x0, y0) and in

the direction .

Example

D f (x0, y0) = D f (x0, y0) = f x (x0, y0) and D f (x0, y0) = D f (x0, y0) = f y (x0, y0).

These are directional derivatives we can obtain with the current knowledge we have available. If

we wish the slopes in other directions, we need to further develop the knowledge we have.

USING THE TANGENT PLANE TO FIND DIRECTIONAL DERIVATES

By definition, D f (x0, y0) is defined as the slope of the tangent line to z = f (x, y) at the point

(x0, y0) and in the direction . In the following diagram we see various tangent lines to a

surface at a given point.

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In the following diagram we can see that all of the tangent lines, irrespective of the direction lie

on the tangent plane to the surface at the point (x0, y0).

Hence, we can conclude that the directional derivative of a surface z = f (x, y) at a point (x0, y0) is

equal to the directional slope of the tangent plane at that point.

Hence with this and data from previous sections, we can conclude the following:

The tangent place to a surface is easily obtained.

If we know the slopes mx and my of a plane, we can obtain the slope of a plane in any

direction.

The directional derivative of a surface is the same as the directional slope of the tangent

plane.

Hence, we have the following general procedure with which we can obtain directional

derivatives.

1. Obtain the tangent plane to the surface z = f (x, y) at the point where the directional derivative s desired.

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2. Obtain the directional slope of the tangent plane in the indicated direction and it will be the same as the directional derivative of the surface.

Example Exercise 6.3.1: Given the surface z = .5x 2 + y

2 , find D f (1, 1).

Solution:

1. Obtain the tangent plane to the surface

Using the point and the two slopes, the tangent plane will be

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The surface can be discarded as the directional derivative of the function will be the

direction slope of the tangent plane we have just obtained.

2. Identify the right triangle whose rise and run are associated with these slopes.

3. Find the change in height associated with the x direction of :

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4. Find the change in height associated with the y direction :

5. Find the overall change in height:

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Taking both components of the change in height, the total difference in height is dz = dzx

+ dzy = 8.

6. Obtain the run for the movement associated with i.e., Dx = 2, Dy = 3

Using Pythagoras and the above right triangle,

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7. Obtain the slope:

Conclusion: The directional slope of the tangent plane is the same as:

Generalization

Given a function x = f (x, y) and a point (x0, y0), we can find D f (x0, y0) through the

following steps:

1. Obtain the tangent plane to the surface:

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Using the point and the two slopes, the tangent plane will be

As we are seeking the differential, the surface can be discarded and we can pay attention

solely to the tangent plane in order to obtain the desired change in height.

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Notice that we have obtained the tangent plane, D f (x0, y0) is equal to the slope on

the tangent plane in the direction .

2. Identify the right triangle whose rise and run are associated with these slopes.

3. Find the rise in the tangent plane

Rise in the x direction

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Rise in the y direction

Total rise:

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4. Obtain the run for

5. The directional slope of the tangent plane is the same as the directional derivative

It is worth nothing that if is a unit vector then