2.7: Linearization and Newton’s Method
Learning Objectives
A student will be able to:
 Approximate a function by the method of linearization.
 Know Newton’s Method for approximating roots of a function.
Linearization: The Tangent Line Approximation
If is a differentiable function at , then the tangent line, , to the curve at is a good approximation to the curve for values of near (Figure 8a). If you “zoom in” on the two graphs, and the tangent line, at the point of tangency, , or if you look at a table of values near the point of tangency, you will notice that the values are very close (Figure 8b).
Since the tangent line passes through point and the slope is , we can write the equation of the tangent line, in pointslope form, as
Solving for ,
So for values of close to , the values of of this tangent line will closely approximate . This gives the approximation
The Tangent Line Approximation (Linearization)
If is a differentiable function at , then the approximation function
is a linearization of near .
Example 1:
Find the linearization of at point .
Solution:
Taking the derivative of ,
we have and
This tells us that near the point , the function approximates the line . As we move away from , we lose accuracy (Figure 9).
Example 2:
Find the linearization of at .
Solution:
Since , and we have
Newton’s Method
When faced with a mathematical problem that cannot be solved with simple algebraic means, such as finding the roots of the polynomial calculus sometimes provides a way of finding the approximate solutions.
Let's say we are interested in computing without using a calculator or a table. To do so, think about this problem in a different way. Assume that we are interested in solving the quadratic equation
which leads to the roots .
The idea here is to find the linearization of the above function, which is a straightline equation, and then solve the linear equation for .
Since
or
We choose the linear approximation of to be near . Since and thus and Using the linear approximation formula,
Notice that this equation is much easier to solve than Setting and solving for , we obtain,
If you use a calculator, you will get As you can see, this is a fairly good approximation. To be sure, calculate the percent difference between the actual value and the approximate value,
where is the accepted value and is the calculated value.
which is less than .
We can actually make our approximation even better by repeating what we have just done not for , but for , a number that is even closer to the actual value of . Using the linear approximation again,
Solving for by setting , we obtain
which is even a better approximation than . We could continue this process generating a better approximation to . This is the basic idea of Newton’s Method.
Here is a summary of Newton’s method.
Newton’s Method
 Guess the first approximation to a solution of the equation . A graph would be very helpful in finding the first approximation (see Figure below).
 Use the first approximation to find the second, the second to find the third and so on by using the recursion relation
Example 3:
Use Newton’s method to find the roots of the polynomial
Solution:
Using the recursion relation,
To help us find the first approximation, we make a graph of . As Figure 11 suggests, set . Then using the recursion relation, we can generate , .
Using the recursion relation again to find , we get
We conclude that the solution to the equation is about .
Multimedia Links
For a video presentation of Newton's method (10.0), see Math Video Tutorials by James Sousa, Newton's method (9:48).
Review Questions
 Find the linearization of at .
 Find the linearization of at .
 Use the linearization method to show that when (much less than ), then . Hint: Let
 Use the result of problem #3, , to find the approximation for the following:
 Without using a calculator, approximate .
 Use Newton’s Method to find the roots of .
 Use Newton’s Method to find the roots of .
Review Answers
 Let , and . Then, by the chain and powers rules:
If , we can use and linearize around the point :

 and
Texas Instruments Resources
In the CK12 Texas Instruments Calculus FlexBook, there are graphing calculator activities designed to supplement the objectives for some of the lessons in this chapter. See http://www.ck12.org/flexr/chapter/9727.