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**11.11** Applications of Taylor

### Polynomials

## Approximating Functions

## by Polynomials

### Approximating Functions by Polynomials

*Suppose that f(x) is equal to the sum of its Taylor series*
*at a:*

*The notation T*_{n}*(x) is used to represent the nth partial sum *
*of this series and we can call it as it the nth-degree Taylor *
*polynomial of f at a. *

Thus

### Approximating Functions by Polynomials

*Since f is the sum of its Taylor series, we know that *
*T*_{n}*(x) f(x) as n* *and so T** _{n}* can be used as an

*approximation to f:*

*f(x) ≈ T*_{n}*(x).*

Notice that the first-degree Taylor polynomial
*T*_{1}*(x) = f(a) + f′(a)(x – a)*

### Approximating Functions by Polynomials

*Notice also that T*_{1} and its derivative have the same values
*at a that f and f*′ have. In general, it can be shown that the
*derivatives of T*_{n}*at a agree with those of f up to and *

*including derivatives of order n.*

To illustrate these ideas let’s
take another look at the graphs
*of y = e*^{x} and its first few Taylor

polynomials, as shown in Figure 1.

**Figure 1**

### Approximating Functions by Polynomials

*The graph of T*_{1} *is the tangent line to y = e** ^{x}* at (0, 1); this

*tangent line is the best linear approximation to e*

*near (0, 1).*

^{x}*The graph of T*_{2} *is the parabola y = 1 + x + x*^{2}/2, and the
*graph of T*_{3} *is the cubic curve y = 1 + x + x*^{2}*/2 + x*^{3}/6, which
*is a closer fit to the exponential curve y = e*^{x}*than T*_{2}.

*The next Taylor polynomial T*_{4} would be an even better
approximation, and so on.

### Approximating Functions by Polynomials

The values in the table give a numerical demonstration of
*the convergence of the Taylor polynomials T*_{n}*(x) to the *

*function y = e*^{x}*. We see that when x = 0.2 the convergence *
*is very rapid, but when x = 3 it is somewhat slower. *

*In fact, the farther x is from 0, the *
*more slowly T*_{n}*(x) converges to e** ^{x}*.
When using a Taylor polynomial

*T*

_{n}*to approximate a function f, we*have to ask the questions:

How good an approximation is it?

### Approximating Functions by Polynomials

*How large should we take n to be in order to achieve a *

desired accuracy? To answer these questions we need to look at the absolute value of the remainder:

*| R*_{n}*(x)| = | f(x) – T*_{n}*(x)|*

There are three possible methods for estimating the size of the error:

**1. If a graphing device is available, we can use it to graph **

*|R*_{n}*(x)| and thereby estimate the error.*

### Approximating Functions by Polynomials

**2. If the series happens to be an alternating series, we can**
use the Alternating Series Estimation Theorem.

**3. In all cases we can use Taylor’s Inequality which says**

that if then

## Example 1

(a) Approximate the function by a Taylor
*polynomial of degree 2 at a = 8.*

(b) How accurate is this approximation when 7 ≤ x ≤ 9?

Solution:

(a)

*Example 1 – Solution*

Thus the second-degree Taylor polynomial is

The desired approximation is

cont’d

*Example 1 – Solution*

*(b) The Taylor series is not alternating when x < 8, so we*
can’t use the Alternating Series Estimation Theorem in
this example.

*But we can use Taylor’s Inequality with n = 2 and a = 8:*

*where | f′″(x)| ≤ M. *

*Because x ≥ 7, we have x*^{8/3} ≥ 7^{8/3 }and so

cont’d

*Example 1 – Solution*

*Therefore we can take M = 0.0021. Also 7 ≤ x ≤ 9, so *
–1 ≤ x – 8 ≤ 1 and |x – 8| ≤ 1.

Then Taylor’s Inequality gives

Thus, if 7 ≤ x ≤ 9, the approximation in part (a) is accurate to within 0.0004.

cont’d

### Approximating Functions by Polynomials

Let’s use a graphing device to check the calculation in

Example 1. Figure 2 shows that the graphs of and
*y = T*_{2}*(x) are very close to each other when x is near 8. *

**Figure 2**

### Approximating Functions by Polynomials

*Figure 3 shows the graph of |R*_{2}*(x)| computed from the *
expression

We see from the graph that

*|R*_{2}*(x)| < 0.0003*
when 7 ≤ x ≤ 9.

Thus the error estimate from graphical methods is slightly better than the error estimate

from Taylor’s Inequality in this case.

**Figure 3**

## Applications to Physics

## Applications to Physics

Taylor polynomials are also used frequently in physics. In order to gain insight into an equation, a physicist often

simplifies a function by considering only the first two or three terms in its Taylor series.

In other words, the physicist uses a Taylor polynomial as an approximation to the function. Taylor’s Inequality can then be used to gauge the accuracy of the approximation.

The following example shows one way in which this idea is used in special relativity.

## Example 3

In Einstein’s theory of special relativity the mass of an
*object moving with velocity v is*

*where m*_{0} *is the mass of the object when at rest and c is the *
speed of light. The kinetic energy of the object is the

difference between its total energy and its energy at rest:

*K = mc*^{2} *– m*_{0}*c*^{2}

## Example 3

*(a) Show that when v is very small compared with c, this*
*expression for K agrees with classical Newtonian *
*physics: K = m*_{0}*v*^{2}.

(b) Use Taylor’s Inequality to estimate the difference in
*these expressions for K when | v | ≤ 100 m/s.*

Solution:

*(a) Using the expressions given for K and m, we get*

cont’d

*Example 3 – Solution*

*With x = –v*^{2}*/c*^{2}*, the Maclaurin series for (1 + x)*^{–1/2} is

*most easily computed as a binomial series with k = *
*(Notice that | x | < 1 because v < c.) Therefore we have*

and

cont’d

*Example 3 – Solution*

*If v is much smaller than c, then all terms after the first *
are very small when compared with the first term. If we
omit them, we get

*(b) If x = –v*^{2}*/c*^{2}*, f(x) = m*_{0}*c*^{2} *[(1 + x)*^{–1/2} *– 1], and M is a*

*number such that | f″(x)| ≤ M, then we can use Taylor’s*
Inequality to write

cont’d

*Example 3 – Solution*

*We have f″(x) = m*_{0}*c*^{2}*(1 + x)*^{–5/2} and we are given that

*| v | ≤ 100 m/s, so*

*Thus, with c = 3 × 10*^{8 }m/s,

*So when |v | ≤ 100 m/s, the magnitude of the error in *
using the Newtonian expression for kinetic energy is at

cont’d