Representing Functions as Power Series: AP Calculus BC Study Guide
Introduction
Greetings, Calculus aficionados! 🎉 Congrats on making it to the final boss of AP Calculus BC: representing functions as power series. Think of power series as the Swiss Army knife of math – infinitely useful and packed with a ton of nifty tools. In this guide, we'll navigate through the tricky terrain of power series and help you master them, so you'll be ready to ace that AP exam. 🚀
Power Series: The Basics
Imagine power series are like an infinite playlist of Taylor polynomials. Each term in a power series is a polynomial, and together, they form an infinite series that can represent a function. It’s like Netflix for functions – endless contents!
A power series around ( r ) can be written as: [ \sum_{n=0}^{\infty} a_n (x  r)^n ] where ( a_n ) are coefficients (real numbers), ( x ) is your variable, and ( r ) is the center of the series.
Important headsup! Memorize these frequently occurring power series for the exam:
 Exponential function ( e^x )
 Sine function ( \sin(x) )
 Cosine function ( \cos(x) )
These series pop up like cameos in Marvel movies, so knowing them will save you loads of time.
Key Power Series to Memorize

Exponential Function ( e^x ): [ e^x = \sum_{n=0}^{\infty} \frac{x^n}{n!} = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \ldots ]

Cosine Function ( \cos(x) ): [ \cos(x) = \sum_{n=0}^{\infty} \frac{(1)^n x^{2n}}{(2n)!} = 1  \frac{x^2}{2!} + \frac{x^4}{4!}  \ldots ]

Sine Function ( \sin(x) ): [ \sin(x) = \sum_{n=0}^{\infty} \frac{(1)^n x^{2n+1}}{(2n+1)!} = x  \frac{x^3}{3!} + \frac{x^5}{5!}  \ldots ]
These heroes of power series will often help you transform a complex function into something more manageable.
Example 1: Power Series Representation
Let's find the power series representation for ( x^2 e^x ) and include the first four nonzero terms and the general term.
We know the power series for ( e^x ): [ e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \ldots ]
Now, since ( x^2 e^x ) is just ( e^x ) multiplied by ( x^2 ): [ x^2 e^x = x^2 \left( 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \ldots \right) = x^2 + x^3 + \frac{x^4}{2!} + \frac{x^5}{3!} + \ldots ]
Here, the first four nonzero terms are: [ x^2, x^3, \frac{x^4}{2!}, \frac{x^5}{3!} ]
The general term: [ \frac{x^{n+2}}{n!} ]
Example 2: Derive from a Power Series
Suppose ( h(x) ) is given by the power series for ( \cos(x) ): [ h(x) = \cos(x) = 1  \frac{x^2}{2!} + \frac{x^4}{4!}  \ldots ]
What is ( h’(x) ) (the derivative)?
We will differentiate term by term: [ h'(x) = x + \frac{x^3}{3!}  \frac{x^5}{5!} + \ldots ]
Notice something cool? This is the negative power series for ( \sin(x) )! Thus, ( h'(x) = \sin(x) ). No cap.
Practice FRQ 2022
Consider this classic AP Calculus BC Free Response question:
Given the function defined by: [ f(x)=x\frac{x^3}{3}+\frac{x^5}{5}\frac{x^7}{7}+\ldots ]
Write the first four nonzero terms and the general term for the power series representing ( f'(x) ).
Take the derivative term by term: [ f'(x)=1x^2+\frac{x^4}{5}\frac{x^6}{7}+\ldots ]
The first four nonzero terms are: [ 1, x^2, \frac{x^4}{5}, \frac{x^6}{7} ]
The general term is: [ (1)^n \frac{x^{2n}}{2n+1} ]
Conclusion
Well done! You’ve reached the end of this unit and with it, the grand finale of AP Calculus BC. 🎊 You now possess the knowledge and tools to dominate your exam. Dive into review sessions and practice problems, and you'll be as ready as a ninja in a math dojo. 🥋📚
Key Terms
 Complex Analysis: Study of functions involving complex numbers.
 Differential Equations: Equations involving derivatives that describe how functions change.
 EpsilonDelta Definition of a Limit: A rigorous definition of limits.
 Fundamental Theorem of Calculus: Relates differentiation and integration.
 Multivariable Calculus: Calculus of functions with multiple variables.
Remember, calculus is like a thrilling mind game with infinite possibilities. Embrace the challenge, use this guide to power up your studies, and you will undoubtedly ace your AP exam! 🚀