Chapter 10: Problem 36
By recognizing each series as a Taylor series evaluated at a particular value of \(x,\) find the sum of each of the following convergent series. $$1-\frac{1}{3 !}+\frac{1}{5 !}-\frac{1}{7 !}+\dots+\frac{(-1)^{n}}{(2 n+1) !}+\cdots$$
Short Answer
Expert verified
The sum of the series is \(\sin(1)\).
Step by step solution
01
Identify the pattern
First, recognize the pattern in the series: the terms alternate in sign and involve odd factorials in the denominator. The given series is: \[ 1 - \frac{1}{3!} + \frac{1}{5!} - \frac{1}{7!} + \cdots = \sum_{n=0}^{\infty} \frac{(-1)^n}{(2n+1)!}. \]
02
Recall the Taylor series for sine
Recall the Taylor series expansion for \(\sin(x)\) centered at 0 is: \[ \sin(x) = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots = \sum_{n=0}^{\infty} \frac{(-1)^n x^{2n+1}}{(2n+1)!}. \]
03
Compare the series and Taylor series
Notice that the given series resembles the Taylor series for \(\sin(x)\), but without the \(x^{2n+1}\) term in each component. That is, for \(\sin(x)\) with \(x = 1\), it becomes:\[ \sin(1) = 1 - \frac{1}{3!} + \frac{1}{5!} - \frac{1}{7!} + \cdots. \]
04
Evaluate the Taylor series at a specific point
Since the series is equivalent to the sum of \(\sin(x)\) with \(x = 1\), the sum of the series is simply \(\sin(1)\). Therefore, \[ \sum_{n=0}^{\infty} \frac{(-1)^n}{(2n+1)!} = \sin(1). \]
05
Conclusion
The given series evaluates to the sine of 1 radian. Hence, the sum of the series is \(\sin(1)\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Convergent Series
A convergent series is a sequence of numbers that approaches a specific value as more terms are added. This means the sum of the series gets closer and closer to a certain number, even though there may be infinitely many terms. A key feature is that despite an infinite number of terms, the overall sum remains finite.
- One example of a convergent series is the geometric series, where the terms form a geometric progression.
- Another example is the alternating harmonic series, where the signs of the terms alternate between positive and negative.
Sine Function
The sine function is a fundamental trigonometric function that describes the y-coordinate of a point on the unit circle as a function of the angle from the positive x-axis. It is an essential component of many mathematical equations and is crucial for modeling periodic phenomena such as sound waves and tidal patterns.
- The sine function is periodic with a period of \(2\cdot\pi\).
- Its range is from -1 to 1, corresponding to the maximum and minimum y-values on the unit circle.
- The sine of an angle is equal to the opposite side divided by the hypotenuse in a right-angled triangle.
Taylor Expansion
Taylor expansion is a method of approximating complex functions with a series of polynomial terms. This technique is extremely useful because it allows us to express a function in terms of its derivatives at a specific point, providing an approximation that becomes increasingly accurate as more terms are included.
- It gives a function’s value at a point \( x \) through the derivatives evaluated at a nearby point, commonly zero.
- Each term of the Taylor series involves derivatives of the function, and these derivatives are divided by factorial terms.
- The Taylor series for \( \sin(x) \) is unique because it only includes odd powers of \( x \).
Factorials
Factorials are a mathematical function that involves multiplying a series of descending natural numbers. They are critical in combinatorics, calculus, and many areas of mathematical computation.
- A factorial of a non-negative integer \( n \) is denoted as \( n! \) and is defined as the product of all positive integers less than or equal to \( n \).
- By definition, \( 0! = 1 \), which is an important base case.
- Factorials grow very rapidly; for instance, \( 5! = 5 \cdot 4 \cdot 3 \cdot 2 \cdot 1 = 120 \).