Chapter 19: Problem 31
Find the first four terms of the indicated expansions by use of the binomial series. $$(1-3 x)^{-2}$$
Short Answer
Expert verified
The first four terms are: \(1 + 6x - 9x^2 - 9x^3\).
Step by step solution
01
Identify the Formula
We need to expand the expression \( (1 - 3x)^{-2} \) using the binomial series. The general form of the binomial series is \((1 + a)^n = 1 + \frac{na}{1!} + \frac{n(n-1)a^2}{2!} + \frac{n(n-1)(n-2)a^3}{3!} + \dots \) for \(|a| < 1\). In our case, \(a = -3x\) and \(n = -2\).
02
Substitute Values
Substitute \(a = -3x\) and \(n = -2\) into the binomial series formula. Our first four terms will be derived from:- The zeroeth term: \(1\)- The first term: \(-2(-3x)\)- The second term: \(-2(-3x)(-3x-1)/(2!)\)- The third term: \(-2(-3x)(-3x-1)(-3x-2)/(3!)\)
03
Calculate First Term
The first term, referring to the constant term in the series, is simply \(1\) (the zero-th power in any expansion).
04
Calculate Second Term
The first power term is \(-2(-3x)\). Simplify this expression:\(-2(-3x) = 6x\).
05
Calculate Third Term
The second power term is formulated as\[-2 \times (-3x) \times (-3x - 1) / 2! \]First, calculate the expression inside:\(-3x (-3x - 1) = 9x^2 + 3x\)Putting it all together:\(-2 \times (9x^2 + 3x) / 2 = -9x^2 - 3x\).
06
Calculate Fourth Term
For the third power term:\[-2 \times (-3x) \times (-3x - 1) \times (-3x - 2) / 3!\]First, expand:\((-3x)(-3x - 1)(-3x - 2) = 27x^3 + 27x^2 + 6x\)Simplify the equation:\(-2 \times (27x^3 + 27x^2 + 6x) / 6 = -9x^3 - 9x^2 - 2x\).
07
Compile the Terms
Combine all the calculated terms together:\[1 + 6x - (9x^2 + 3x) - (9x^3 + 9x^2 + 2x)\]Combine like terms:\[1 + 6x - 9x^2 - 9x^3\]
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Binomial Expansion
The process of binomial expansion involves expanding a binomial expression raised to any power, using a binomial series. The binomial series provides a systematic way to break down complex expressions, making them easier to understand and compute. This is particularly useful when dealing with expressions of the form \((1 + a)^n\).
To perform a binomial expansion for a negative power, as in our example \((1-3x)^{-2}\), we rely on the generalized binomial series formula. This formula helps us express the original binomial in terms of a series, where each term can be calculated using coefficients generated by the formula.
To perform a binomial expansion for a negative power, as in our example \((1-3x)^{-2}\), we rely on the generalized binomial series formula. This formula helps us express the original binomial in terms of a series, where each term can be calculated using coefficients generated by the formula.
- The expansion begins with 1, representing the constant term.
- Subsequent terms involve progressively higher powers of the variable, multiplied by coefficients.
Series Expansion
Series expansion is a mathematical technique used to express a function as a sum of terms. Each term represents a progressively higher power of the variable. This is a powerful tool in calculus and algebra, providing a way to approximate complex functions.
In our exercise, we specifically deal with the binomial series expansion. Starting from the general expression \((1 + a)^n\), series expansion allows us to rewrite this as an infinite series of terms. Importantly, we only consider the first few terms for approximations or calculations involving simple expressions:
In our exercise, we specifically deal with the binomial series expansion. Starting from the general expression \((1 + a)^n\), series expansion allows us to rewrite this as an infinite series of terms. Importantly, we only consider the first few terms for approximations or calculations involving simple expressions:
- The zero-th term remains constant.
- Higher terms involve sequential multipliers of the variable \(a\) and the power \(n\).
Mathematical Series
A mathematical series is essentially the sum of the terms in a sequence. With each additional term, the series grows closer to approximating a particular function or value. This concept is widely used in various fields of mathematics for analyzing functions and approximating values.
The binomial series is a specific type of mathematical series. It helps us transform power expressions like \((1 - 3x)^{-2}\) into a series format involving sum of terms such as \(1 + 6x - 9x^2 - 9x^3\). This kind of representation is more practical than directly working with powers, especially when dealing with functions.
The binomial series is a specific type of mathematical series. It helps us transform power expressions like \((1 - 3x)^{-2}\) into a series format involving sum of terms such as \(1 + 6x - 9x^2 - 9x^3\). This kind of representation is more practical than directly working with powers, especially when dealing with functions.
- Each term in a mathematical series builds upon the previous term.
- This cumulative process helps with convergence and practical computation.
Polynomial Expansion
Polynomial expansion involves breaking down a binomial expression into a series of polynomial terms. This is essential in algebra when manipulating expressions or solving equations.
In polynomial expansion, the expression \((1 - 3x)^{-2}\) is expanded into a series of terms that can be added or subtracted to form a new polynomial: \(1 + 6x - 9x^2 - 9x^3\).
In polynomial expansion, the expression \((1 - 3x)^{-2}\) is expanded into a series of terms that can be added or subtracted to form a new polynomial: \(1 + 6x - 9x^2 - 9x^3\).
- Each of these terms is a polynomial component.
- Polynomial terms can be combined or simplified to make calculations simpler.