Polynomial approximation of functions (part 4)

Khan Academy
28 Apr 200809:59
EducationalLearning
32 Likes 10 Comments

TLDRIn this educational video, the presenter explores the Maclaurin series representation of the cosine function, comparing it to the series for the exponential function e^x. Starting with the first derivative of cosine, the video demonstrates the pattern of derivatives alternating between sine and cosine, ultimately leading to a Maclaurin series for cosine that involves even powers of x and alternating signs. The presenter hints at the intriguing relationship between these series and their implications in understanding the connections between e, cosine, sine, and other mathematical constants.

Takeaways
  • 📚 The video discusses the Maclaurin series representation of functions, starting with the cosine function.
  • 🔍 The first derivative of cos(x) is found to be -sin(x), and the second derivative is -cos(x), indicating a repeating pattern in the derivatives.
  • 🔄 The derivatives of cosine alternate between even and odd functions, repeating every four derivatives.
  • 📈 The Maclaurin series is a specific case of the Taylor series, centered at x=0, and is used to approximate functions around a point.
  • 🌐 The script demonstrates the process of finding the Maclaurin series for cos(x) by evaluating the derivatives at x=0 and observing a pattern.
  • 🔢 The values of the derivatives at x=0 for cos(x) are 1, 0, -1, 0, 1, 0, and so on, alternating between 1 and -1 with zeros in between.
  • 📝 The Maclaurin series for cos(x) is constructed using these derivative values, with terms involving x raised to even powers and alternating signs.
  • 📉 The script simplifies the series by factoring out the even powers and the alternating signs, resulting in a neat expression for cos(x).
  • 🤔 The video assumes that the Maclaurin series converges to cos(x) over its entire domain, a point that may be proven in future lessons.
  • 🔗 The representation of cos(x) derived resembles part of the series for e^x, highlighting a connection between different functions.
  • 🚀 The video concludes with a teaser for the next lesson, which will explore the Maclaurin series for sin(x) and further connections between mathematical functions.
Q & A

  • What is the Maclaurin series representation of e^x?

    -The Maclaurin series representation of e^x is a specific case of the Taylor series where the function is expanded around x=0. It includes all the derivatives of the function evaluated at x=0, multiplied by x raised to the power of n, divided by n factorial, for n starting from 0 to infinity.

  • What is the first derivative of the cosine function?

    -The first derivative of the cosine function, denoted as f'(x), is equal to -sin(x).

  • What is the second derivative of the cosine function?

    -The second derivative of the cosine function, denoted as f''(x), is equal to -cos(x), which is obtained by differentiating -sin(x).

  • How does the pattern of derivatives for the cosine function repeat?

    -The pattern of derivatives for the cosine function repeats every four derivatives: cos(x), -sin(x), -cos(x), sin(x), and then it starts over with cos(x) again.

  • What is the value of the cosine function at x=0?

    -The value of the cosine function at x=0 is 1, since cos(0) = 1.

  • What is the value of the first derivative of the cosine function at x=0?

    -The value of the first derivative of the cosine function at x=0 is 0, because sin(0) = 0, and thus -sin(0) = 0.

  • How are the even and odd terms of the Maclaurin series for cosine(x) represented?

    -The even terms of the Maclaurin series for cosine(x) are represented by the formula involving x to the power of 2n over (2n)!, where n starts from 0. The odd terms are not present because the series alternates between positive and negative values, effectively canceling out the odd powers of x.

  • What is the significance of the alternating signs in the Maclaurin series for cosine(x)?

    -The alternating signs in the Maclaurin series for cosine(x) are crucial as they reflect the periodic nature of the cosine function. The negative sign in front of the even powers of x ensures that the series correctly represents the cosine wave's peaks and troughs.

  • How does the Maclaurin series for cosine(x) resemble the series for e^x?

    -The Maclaurin series for cosine(x) resembles the series for e^x in that both are infinite series that involve powers of x divided by factorial terms. The main difference is that the cosine series includes only even powers of x and alternates signs, while e^x has all powers of x and does not alternate signs.

  • What is the general formula for the nth term of the Maclaurin series for a function?

    -The general formula for the nth term of the Maclaurin series for a function f(x) is f^(n)(0) * x^n / n!, where f^(n)(0) is the nth derivative of the function evaluated at x=0.

  • What assumption is made about the convergence of the Maclaurin series for cosine(x)?

    -The assumption made about the convergence of the Maclaurin series for cosine(x) is that it converges to the actual value of the cosine function over its entire domain. This is an assumption that may be proven in the future with the right mathematical tools.

Outlines
00:00
📚 Derivative Patterns and Maclaurin Series for Cosine Function

This paragraph introduces the concept of deriving the cosine function and its subsequent derivatives to explore the pattern that emerges. It begins by defining the function f(x) as cosine of x and calculating its first, second, and third derivatives, revealing a repeating cycle of sine and cosine functions with alternating signs. The focus then shifts to the Maclaurin series representation, a specific case of the Taylor series, which is centered around evaluating the derivatives at x equals zero. The process involves identifying the values of the cosine function and its derivatives at zero, which alternate between 1, 0, and -1, setting the stage for the Maclaurin series expansion.

05:01
🔍 Simplifying the Maclaurin Series for Cosine

Building upon the previous discussion, this paragraph delves deeper into the Maclaurin series for the cosine function. It simplifies the series by eliminating the zero-valued terms and focusing on the pattern of even powers of x, each term being multiplied by alternating signs starting with a positive sign. The series is then expressed in a more compact form using a summation notation that captures the essence of the cosine function's behavior over its entire domain. The paragraph concludes by drawing a parallel between the cosine function's Maclaurin series and that of the exponential function, e^x, highlighting the resemblance in their structures, with the key difference being the presence of odd powers and constant signs in the latter. The summary sets the stage for further exploration of the sine function in the subsequent video.

Mindmap
Keywords
💡Maclaurin series
The Maclaurin series is a special case of the Taylor series, expanded around x = 0. In the video, it's used to represent functions such as e^x, cosine, and sine. The process involves finding the function's value and its derivatives at zero.
💡Cosine
Cosine is a trigonometric function that describes the ratio of the adjacent side to the hypotenuse in a right triangle. The video explores the Maclaurin series representation of cosine(x), identifying a pattern in the derivatives and their values at x = 0.
💡Derivative
A derivative represents the rate at which a function is changing at any given point. In the video, the derivatives of cosine(x) are calculated to construct its Maclaurin series, revealing a repeating pattern.
💡Factorial
A factorial, denoted as n!, is the product of all positive integers up to n. It's used in the denominators of the terms in the Maclaurin series to normalize the growth of the polynomial terms, ensuring convergence.
💡Pattern
In the context of the video, a pattern refers to the regular and predictable behavior observed in the derivatives of cosine(x). The pattern alternates between zero and values of 1 or -1, which helps in constructing the series.
💡Alternating series
An alternating series is a series whose terms alternate in sign. The video demonstrates that the Maclaurin series for cosine(x) is an alternating series, switching between positive and negative values.
💡Sine
Sine is another trigonometric function, representing the ratio of the opposite side to the hypotenuse. The video references sine(x) when calculating the derivatives of cosine(x), as they are interconnected.
💡Convergence
Convergence refers to the property of a series to approach a limit as more terms are added. The video assumes that the Maclaurin series for cosine(x) converges to the actual function across its domain.
💡Taylor series
The Taylor series generalizes the Maclaurin series to expand functions around any point, not just zero. The video mentions this broader context to help explain the specific use of the Maclaurin series.
💡Exponential function (e^x)
The exponential function e^x is used as a comparison in the video to highlight similarities and differences with the Maclaurin series of cosine(x). The key difference noted is the absence of alternating signs and inclusion of all terms for e^x.
Highlights

Introduction to the Maclaurin series representation of e^x and its application to other functions.

Derivation of the first, second, and third derivatives of the cosine function, revealing a repeating pattern.

Identification of the cosine function's derivatives' pattern: alternating between 1, 0, -1, 0, and repeating.

Explanation of the Maclaurin series as a specific case of the Taylor series, focusing on the function's derivatives at x=0.

Calculation of the Maclaurin series coefficients for the cosine function, showing a clear pattern of 1, 0, -1, 0.

Demonstration of the Maclaurin series convergence over the entire domain of the function, an assumption made for the series.

Expression of the cosine function as an infinite sum of its derivatives evaluated at x=0, illustrating the Maclaurin series formula.

Observation that the Maclaurin series for cosine resembles the series for e^x, with the difference being the sign alternation and even powers.

Simplification of the Maclaurin series for cosine by focusing on even powers of x and alternating signs.

Introduction of the alternating sign pattern using (-1)^(n+1) in the Maclaurin series for cosine.

Final expression of the cosine function as a sum of even powers of x with alternating signs, resembling part of the e^x representation.

Discussion of the relationship between the Maclaurin series for cosine and sine, and their connection to e, i, and pi.

Preview of the next video's content, which will cover the Maclaurin series for the sine function and its integration with the cosine series.

Emphasis on the intuitive understanding of the Maclaurin series as a function that matches the original function and its derivatives at a chosen point.

Note on the need for future tools to prove the convergence of the Maclaurin series for cosine, currently taken as an assumption.

The Maclaurin series as a powerful mathematical tool for approximating functions and understanding their behavior.

Transcripts

Browse More Related Video

Maclaurin series of sin(x) | Series | AP Calculus BC | Khan Academy

Maclaurin series of cos(x) | Series | AP Calculus BC | Khan Academy

Maclaurin series of e_ | Series | AP Calculus BC | Khan Academy

Polynomial approximation of functions (part 6)

Taylor Series to Memorize

Calculus Chapter 1 Lecture 2 Exponentials

Rate This

5.0 / 5 (0 votes)

Thanks for rating:

Related Tags
Maclaurin SeriesCosine FunctionDerivativesMathematicsEducationalTrigonometrySeries ExpansionCalculusVideo TutorialFunction Analysis