# Lec 39 | MIT 18.01 Single Variable Calculus, Fall 2007 | Summary and Q&A

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September 9, 2009
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MIT OpenCourseWare
Lec 39 | MIT 18.01 Single Variable Calculus, Fall 2007

## TL;DR

Learn about power series and their applications in calculus, including calculating the power series for functions like sin(x) and ln(1+x).

## Key Insights

• ✊ Power series are a way to represent functions as a sum of integral powers of x.
• ✊ The radius of convergence determines where the power series converges or diverges.
• ✊ Power series can be multiplied, differentiated, integrated, and substituted to find power series expansions for different functions.
• ☺️ Power series expansions can be used to calculate the values of functions at different x values with a desired level of accuracy.

## Transcript

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### Q: What is a power series, and how is it different from a polynomial?

A power series is a representation of a function as a sum of integral powers of x, with coefficients that determine the contribution of each power. While polynomials are a special type of power series, power series can extend to infinite terms and have a radius of convergence.

### Q: What is the radius of convergence and why is it important?

The radius of convergence is a number R between 0 and infinity, inclusive, where the power series converges for absolute values of x less than R and diverges for absolute values of x greater than R. It is important because it determines the range of x values where the power series is a valid representation of the function.

### Q: How are power series obtained for functions like sin(x) and ln(1+x)?

Power series for trigonometric functions like sin(x) can be derived by finding the derivatives of the function and evaluating them at x = 0. For ln(1+x), the power series is obtained by substituting the power series expansion of 1/(1+x) into the integral expression for ln(1+x).

### Q: Can power series be used for functions other than polynomials and trigonometric functions?

Yes, power series can be used for a variety of functions, including exponential functions, logarithmic functions, and even more complex functions. As long as a function has a reasonable expression, it can be represented as a power series.

## Summary & Key Takeaways

• Power series are a way of representing functions as a sum of integral powers of x.

• Power series behave similarly to polynomials, with a radius of convergence determining where the series converges or diverges.

• Power series can be multiplied, differentiated, integrated, and substituted to find power series expansions for different functions.