# 7 | FRQ (No Calculator) | Practice Sessions | AP Calculus BC

TLDRIn this AP Daily Practice Session, Bryan and Tony delve into the intricacies of free response problems involving series and Taylor polynomials for the upcoming AP Calculus BC exam. They methodically explain how to find derivatives at specific points, apply the second derivative test for local maxima and minima, and determine the interval of convergence for a geometric series. The session also covers the transformation of a power series into a rational expression and concludes with a clear demonstration of using the Lagrange error bound to estimate the error in approximating a function's value with a Taylor polynomial.

###### Takeaways

- ๐ The session focuses on tackling free response problems related to series and Taylor polynomials for the AP Calculus BC exam preparation.
- ๐ข A third-degree Taylor polynomial is given for a function f about x=5, which is used to find f'(5) and f''(5).
- ๐ The absence of a linear term in the polynomial indicates that f'(5) equals 0, suggesting a critical value at x=5.
- ๐ The second derivative test is used to determine that there is a local minimum at x=5 since f''(5) is positive.
- ๐ The problem-solving process involves understanding the structure of Taylor polynomials and applying calculus concepts learned earlier in the course.
- ๐ The concept of a fourth-degree Taylor polynomial is introduced, and a method to find the fourth derivative of f at 5 without knowing the original function is discussed.
- ๐ A Taylor series for f''(x) is constructed, and it is determined that the series converges on the interval (4.5, 5.5) based on the geometric series test.
- ๐ The second derivative function is expressed as a rational expression, which allows for the calculation of f'(x) using integration.
- ๐งฎ The Lagrange error bound is explained and applied to show that the absolute value of the error at x=5.5 is less than 1/20.
- ๐ The session emphasizes the importance of understanding the underlying concepts and applying problem-solving strategies with confidence.
- ๐ The video concludes with encouragement for continued study and preparation for the AP exam, highlighting the students' potential for success.

###### Q & A

### What is the main topic of the AP Daily Practice Session in the transcript?

-The main topic of the session is tackling free response problems covering series and Taylor polynomials for the AP Calculus BC exam preparation.

### What is the degree of the Taylor polynomial discussed in the first part of the transcript?

-The Taylor polynomial discussed is of the third degree.

### How is the Taylor polynomial for a function f about x equals 5 defined in the transcript?

-The Taylor polynomial is defined as negative 3, plus 10/3 times (x - 5)^2, minus 20/9 times (x - 5)^3.

### What are the steps to find f prime of 5 and f double prime of 5 from the Taylor polynomial?

-To find f prime of 5, we look at the coefficient of the linear term in the polynomial, which is 0 since there is no linear term. This implies f prime of 5 equals 0. To find f double prime of 5, we look at the coefficient of the quadratic term, which is 10/3 from the given polynomial.

### What does the value of f prime of 5 indicate about the graph of f at x equals 5?

-The value of f prime of 5 being 0 indicates that there is a critical value at x equals 5, which could potentially be a local maximum, local minimum, or neither.

### How is the second derivative test used to determine the nature of the critical point at x equals 5?

-The second derivative test is used by evaluating the sign of f double prime of 5. Since f double prime of 5 is positive (20/3), the graph is concave up at x equals 5, indicating a local minimum.

### What is the task in the section where Taylor polynomials are adapted for part B of the transcript?

-The task is to show that the fourth derivative of f at 5 is 16, given that the fourth degree Taylor polynomial for f(x) about x equals 5, and p sub 4 of 8 is 21.

### How is the fourth derivative of f at 5 found in the transcript?

-The fourth derivative is found by setting up an equation using the given values and the structure of the Taylor polynomial. After simplifying and solving the equation, it is determined that the fourth derivative of f at 5 is indeed 16.

### What is the power series for f double prime about x equals 5 mentioned in part C?

-The power series for f double prime about x equals 5 is a geometric series that converges on a certain interval. The series is derived from the second derivative of f at x equals 5.

### How is the interval of convergence for the geometric series found?

-The interval of convergence is found by applying the rule for geometric series convergence, which states that the absolute value of the common ratio R must be less than 1. By calculating the common ratio and setting up an inequality, the interval is determined to be between 9/2 and 11/2.

### What is the rational expression derived for the second derivative function in part D?

-The rational expression derived for the second derivative function is 20/(6x - 27) + C, where C is a constant determined by the fact that f prime of 5 equals 0.

### How is the function f prime of x found using the rational expression in part D?

-The function f prime of x is found by taking the antiderivative of the rational expression for the second derivative. This results in an expression involving a natural logarithm and a constant term, which is simplified using the fact that f prime of 5 equals 0.

### What is the task in the Lagrange error part of the transcript?

-The task is to use the Lagrange error bound to show that the absolute value of the difference between the function at x equals 5.5 and its third degree Taylor polynomial at the same point is less than 1/20, given that the fourth derivative of f satisfies an inequality on a closed interval.

###### Outlines

##### ๐ Introduction to AP Calculus FRQ on Series and Taylor Polynomials

This paragraph introduces the seventh session of the AP Daily Practice Sessions, focusing on free response questions related to series and Taylor polynomials. Bryan Passwater and Tony Record aim to help students feel more confident about the upcoming AP Calculus BC exam. The session will cover Taylor polynomials, Taylor series, and the Lagrange error, with a particular emphasis on solving problems with 100% confidence. The first problem involves a third-degree Taylor polynomial for a function f about x equals 5, and the task is to find f prime of 5 and f double prime of 5. The discussion then explores whether the graph of f has a local maximum, local minimum, or neither at x equals 5, providing a detailed explanation of the reasoning behind the conclusions.

##### ๐ข Solving for Fourth Degree Taylor Polynomial and Derivatives

In this paragraph, the focus shifts to solving for a fourth degree Taylor polynomial for a function f about x equals 5, given that p sub 4 of 8 is 21. The challenge is to demonstrate that the fourth derivative of f at 5 is 16. The explanation involves understanding the structure of the Taylor polynomial and using the given information to isolate and solve for the required derivative. The process involves basic arithmetic and algebraic manipulation to arrive at the final answer, showcasing the interplay between different parts of calculus and the clever problem-solving approach required.

##### ๐ Determining the Convergence Interval of a Geometric Series

This section delves into part C of the problem, which involves determining the interval of convergence for a geometric series derived from the second derivative of a function f about x equals 5. The series is shown to be geometric, and the task is to find the interval where the series converges. The explanation involves understanding the properties of geometric series, specifically that they converge if the absolute value of the common ratio R is less than 1. By applying this rule and performing algebraic calculations, the interval of convergence is determined to be between 9/2 and 11/2. The paragraph also touches on the concept of writing the second derivative function as a rational expression, which is crucial for the next part of the problem.

##### ๐งฎ Applying Lagrange Error Bound to Approximate Function Values

The final paragraph discusses the application of the Lagrange error bound to estimate the error in approximating the value of a function at x equals 5.5 using a third-degree Taylor polynomial. The problem states that the fourth derivative of f satisfies an inequality, and the goal is to show that the absolute value of the difference between the function and its approximation at 5.5 is less than 1/20. The explanation involves understanding the structure of the Lagrange error bound and using the given maximum value of the fourth derivative to calculate the error. The paragraph emphasizes that the Lagrange error bound does not have to be as challenging as students might think and concludes with an encouragement for students to continue their studies and prepare for the AP exam.

###### Mindmap

###### Keywords

##### ๐กTaylor Polynomial

##### ๐กFree Response Problem

##### ๐กLagrange Error

##### ๐กDerivatives

##### ๐กSecond Derivative Test

##### ๐กGeometric Series

##### ๐กCritical Values

##### ๐กLocal Maximum/Minimum

##### ๐กRational Expression

##### ๐กAntiderivative

###### Highlights

Introduction to the seventh AP Daily Practice Session focusing on AP Calculus BC exam preparation.

Discussion on tackling free response problems involving series and Taylor polynomials.

Explanation of a third-degree Taylor polynomial for a function f about x equals 5.

Procedure to find f prime of 5 and f double prime of 5 using the Taylor polynomial.

Determination of local maxima and minima at x equals 5 using the first and second derivatives.

Illustration of the second derivative test to assess the concavity of the graph.

Transition to another adaptation of Taylor polynomials with a focus on the fourth degree.

Demonstration of how to find the fourth derivative of f at 5 given p sub 4 of 8 is 21.

Elucidation on the structure of the Taylor polynomial and its application to find specific term values.

Use of the Taylor series for f double prime to determine the interval of convergence for a geometric series.

Derivation of the third derivative from the given polynomial coefficient and its implications.

Construction of the power series for f double prime and its identification as geometric.

Application of the geometric series convergence rule to find the interval of convergence.

Transformation of the second derivative function into a rational expression using the sum of a geometric series.

Process of finding f prime of x by integrating the rational expression obtained from the geometric series.

Determination of the constant C in the antiderivative by using the known f prime at x equals 5.

Simplification of the obtained function for f prime and its final form using logarithmic properties.

Introduction to the Lagrange error bound and its application to estimate the error at x equals 5.5.

Proof that the absolute value of the error is less than 1/20 using the Lagrange error bound.

Conclusion of the session with encouragement for continued study and preparation for the AP exam.

###### Transcripts

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