Radius and Interval of Convergence of Power Series: AP Calculus BC Study Guide
Introduction
Greetings, fellow calculus crusaders! Prepare to delve into the realm of power series, a mystical land where infinite sums and magical xvalues converge. This journey is exclusive to AP Calculus BC students—Calculus AB folks, you can sit this one out. 😜 Let's demystify power series, radius of convergence, and intervals of convergence. Ready? Hold on to your protractors!
What’s a Power Series?
Imagine a neverending string of numbers trying to be a function. That’s a power series! Mathematically, it’s expressed as:
[ \sum_{n=0}^{\infty} a_n (x  r)^n ]
Here, ( n ) is like your series hype man, always a nonnegative integer. 🥳 The ( a_n ) values are the coefficients, and ( r ) is the center point of our series. For instance, a power series centered at ( x = 3 ) would look like ( \sum_{n=0}^{\infty} a_n (x  3)^n ).
Radius and Interval of Convergence
Now, you might be wondering, "Where does this infinite party actually make sense?" That’s where the radius and interval of convergence come in! The radius of convergence (RoC) tells us how far from ( r ) (the center) our series stays in tune. The interval of convergence (IoC) is the range of xvalues where this convergence happens.
Ratio Test: Your New BFF
To find the RoC, we wield the mighty Ratio Test. It's like the bouncer who decides if our series gets to party or is sent packing. For a series ( \sum a_n ), the Ratio Test uses:
[ L = \lim_{n \to \infty} \left \frac{a_{n+1}}{a_n} \right ]
Here are the rules:
 If ( L < 1 ), the series converges.
 If ( L > 1 ), the series diverges.
 If ( L = 1 ), the test is inconclusive (you might as well flip a coin).
Power Series Practice Question
Let’s see an example in action. We’ll find the interval, radius of convergence, and the center of convergence for this series:
[ \sum_{n=0}^{\infty} \frac{2^n}{n} (4x  8)^n ]
First, break down our terms:
 ( a_n = \frac{2^n}{n} (4x  8)^n )
 ( a_{n+1} = \frac{2^{n+1}}{n+1} (4x  8)^{n+1} )
Using the Ratio Test: [ L = \lim_{n \to \infty} \left \frac{a_{n+1}}{a_n} \right = \lim_{n \to \infty} \left \frac{2^{n+1}(4x  8)^{n+1}}{(n+1)} \cdot \frac{n}{2^n (4x  8)^n} \right ]
After canceling out common terms, we get: [ L = \lim_{n \to \infty} \left \frac{2 \cdot n (4x  8)}{n+1} \right = 4x  8 \cdot 2 ]
For convergence, ( L < 1 ): [ 2 (4x  8) < 1 ] [ 8x  16 < 1 ] [ x  2 < \frac{1}{8} ]
This tells us the radius of convergence (RoC) is (\frac{1}{8}), centered at ( x = 2 ).
Finding the Interval of Convergence
So far, we know: [ x  2 < \frac{1}{8} ] Breaking this into an interval: [ \frac{1}{8} < x  2 < \frac{1}{8} ] [ 2  \frac{1}{8} < x < 2 + \frac{1}{8} ] [ \frac{15}{8} < x < \frac{17}{8} ]
Next up, we test the endpoints ( x = \frac{15}{8} ) and ( x = \frac{17}{8} ):

At ( x = \frac{15}{8} ), our series becomes: [ \sum_{n=0}^{\infty} \frac{2^n}{n} \left(4 \left(\frac{15}{8}\right)  8\right)^n = \sum_{n=0}^{\infty} \frac{(1)^n}{n} ] This is the alternating harmonic series, which converges.

At ( x = \frac{17}{8} ), our series becomes: [ \sum_{n=0}^{\infty} \frac{2^n}{n} \left(4 \left(\frac{17}{8}\right)  8\right)^n = \sum_{n=0}^{\infty} \frac{1}{n} ] This is the harmonic series, which diverges.
Thus, the interval of convergence is: [ \left[ \frac{15}{8}, \frac{17}{8} \right) ] Or in decimal form: [ 1.875 \leq x < 2.125 ]
Summing Up Power Series
To summarize:
 A power series represents a function on an interval where it converges.
 The ratio test helps determine the radius of convergence.
 Test both endpoints to nail down the interval of convergence.
When you encounter a power series problem, remember:
 Apply the ratio test to find the radius of convergence.
 Check the endpoints to determine the interval of convergence.
 Keep your math toolkit handy!
Mastering this topic combines your skills from various math concepts, so practice makes perfect. And remember, every integral step leads to infinite wisdom. Good luck, math magicians! 🎈
Key Terms to Review
 Endpoints: The two extreme points of a line segment or interval.
 Interval of Convergence: The range of xvalues for which a power series converges.
 Power Series: An infinite series that represents a function as an infinite polynomial expression.
 Ratio Test: A test used to determine whether an infinite series converges by examining the limit of the ratio of consecutive terms.
 Taylor Series: An expansion of a function into an infinite sum of terms, each term represents contributions from different derivatives.