Chapter 11: Problem 8
Find Taylor's formula with remainder (11.45) for the given \(f(x), c,\) and \(n\). $$ f(x)=\cos x, \quad c=\pi / 4, \quad n=3 $$
Short Answer
Expert verified
The Taylor formula with remainder is \( T_3(x) + R_3(x) \) for \( f(x) = \cos x \).
Step by step solution
01
Understand the Taylor Series Expansion
Taylor's formula for a function \( f(x) \) expanded around a point \( c \) up to order \( n \) is given by:\[T_n(x) = f(c) + f'(c)(x-c) + \frac{f''(c)}{2!}(x-c)^2 + \frac{f'''(c)}{3!}(x-c)^3 + \cdots + \frac{f^{(n)}(c)}{n!}(x-c)^n\]The remainder is:\[R_n(x) = \frac{f^{(n+1)}(\xi)}{(n+1)!}(x-c)^{n+1}\]where \( \xi \) is some number between \( c \) and \( x \).
02
Find Derivatives and Evaluate at \( c \)
First, calculate the derivatives of \( f(x) = \cos x \) up to the fourth derivative, since \( n=3 \).- \( f(x) = \cos x \) and \( f(c) = \cos(\pi/4) = \frac{\sqrt{2}}{2} \)- \( f'(x) = -\sin x \), \( f'(c) = -\sin(\pi/4) = -\frac{\sqrt{2}}{2} \)- \( f''(x) = -\cos x \), \( f''(c) = -\cos(\pi/4) = -\frac{\sqrt{2}}{2} \)- \( f'''(x) = \sin x \), \( f'''(c) = \sin(\pi/4) = \frac{\sqrt{2}}{2} \)
03
Construct the Third-Degree Taylor Polynomial
Substitute the values of the derivatives into the Taylor polynomial formula:\[T_3(x) = \frac{\sqrt{2}}{2} + \left(-\frac{\sqrt{2}}{2}\right)(x-\frac{\pi}{4}) + \frac{-\frac{\sqrt{2}}{2}}{2!}(x-\frac{\pi}{4})^2 + \frac{\frac{\sqrt{2}}{2}}{3!}(x-\frac{\pi}{4})^3\]
04
Identify the Remainder Term
For \( n = 3 \), the remainder \( R_3(x) \) is:\[R_3(x) = \frac{f^{(4)}(\xi)}{4!}(x-\frac{\pi}{4})^4 = \frac{\cos(\xi)}{24}(x-\frac{\pi}{4})^4\]Since \( f^{(4)}(x) = f(x) = \cos x \). The point \( \xi \) is some point between \( x \) and \( \frac{\pi}{4} \).
05
Write the Complete Formula with Remainder
The Taylor formula with the remainder for \( f(x) = \cos x \), \( c = \frac{\pi}{4} \), up to the third degree is:\[\cos x = T_3(x) + R_3(x)\]Where:\[T_3(x) = \frac{\sqrt{2}}{2} - \frac{\sqrt{2}}{2}(x-\frac{\pi}{4}) - \frac{\sqrt{2}}{4}(x-\frac{\pi}{4})^2 + \frac{\sqrt{2}}{12}(x-\frac{\pi}{4})^3\]\[R_3(x) = \frac{\cos(\xi)}{24}(x-\frac{\pi}{4})^4\]
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Remainder Term in Taylor Series
In Tayloer Series, the remainder term plays a crucial role in understanding how closely a Taylor polynomial approximates a function. When you expand a function around a point, say \( c \), the remainder term, also known as the error, measures the difference between the actual function value and the Taylor polynomial’s value at a certain point \( x \). It represents the amount by which the polynomial approximation may miss the function.
For a function \( f(x) \), the Taylor series around the point \( c \) up to the \( n \)th order is given by the polynomial \( T_n(x) \). The remainder term is provided by:
For a function \( f(x) \), the Taylor series around the point \( c \) up to the \( n \)th order is given by the polynomial \( T_n(x) \). The remainder term is provided by:
- \( R_n(x) = \frac{f^{(n+1)}(\xi)}{(n+1)!}(x-c)^{n+1} \)
Taylor Polynomial
A Taylor polynomial is essentially a finite sum of terms calculated from the values and derivatives of a function at a specific point. It serves as an approximation of a function, particularly around a chosen point \( c \). This polynomial helps students break down complex functions into simpler, more manageable pieces.
For a function \( f(x) \), its Taylor polynomial around a point \( c \) up to the \( n \)th order is given by:
For a function \( f(x) \), its Taylor polynomial around a point \( c \) up to the \( n \)th order is given by:
- \( T_n(x) = f(c) + f'(c)(x-c) + \frac{f''(c)}{2!}(x-c)^2 + \cdots + \frac{f^{(n)}(c)}{n!}(x-c)^n \)
Calculus Problem Solving
Calculus problem solving often involves several key steps and methodologies, particularly when dealing with Taylor series. Understanding the full scope of a problem, performing correct calculations, and interpreting results are vital skills to develop.
Here’s a general approach to problem solving using calculus:
Here’s a general approach to problem solving using calculus:
- **Identify the Function and Point:** Determine the function you are approximating and the point about which you are expanding the series, which in this case is \( c = \frac{\pi}{4} \).
- **Calculate Necessary Derivatives:** Find the derivatives of the function up to the required order as it affects the Taylor polynomial and the remainder.
- **Construct the Taylor Polynomial:** Use the derivatives to form the Taylor polynomial, which is used to approximate the function.
- **Determine the Remainder Term:** Understand and calculate the remainder term that quantifies the approximation error.