Chapter 10: Problem 79
The series in Exercise 5 can also be written as $$ \sum_{n=1}^{\infty} \frac{1}{(n+1)(n+2)} \text { and } \sum_{n=1}^{\infty} \frac{1}{(n+3)(n+4)} $$ Write it as a sum beginning with (a) \(n=-2,\) (b) \(n=0\) (c) \(n=5\)
Short Answer
Expert verified
(a) Start at \(n=-2\, \sum_{m=-2}^{\infty}\frac{1}{(m+4)(m+5)}\), (b) Start at \(n=0\, \sum_{m=0}^{\infty}\frac{1}{(m+4)(m+5)}\), (c) Start at \(n=5\, \sum_{m=5}^{\infty}\frac{1}{(m-3)(m-2)}\).
Step by step solution
01
Understanding the Series
We need to manipulate the indices of the given series (a) \( \sum_{n=1}^{\infty} \frac{1}{(n+1)(n+2)} \) and (b) \( \sum_{n=1}^{\infty} \frac{1}{(n+3)(n+4)} \) so they start at different values of \( n \).
02
Change of Index for (a) to Start at \(n=-2\)
For \( \sum_{n=1}^{\infty} \frac{1}{(n+1)(n+2)} \), we need to subtract 3 from the index: Substitute \( n = m + 3 \) where \( m = -2 \), thus \( n+1 = (m+3)+1 = m+4 \, \) and \( n+2 = (m+3)+2 = m+5 \). The new series is \( \sum_{m=-2}^{\infty} \frac{1}{(m+4)(m+5)} \).
03
Change of Index for (a) to Start at \(n=0\)
For \( \sum_{n=1}^{\infty} \frac{1}{(n+1)(n+2)} \), we need to subtract 1 from the index: Substitute \( n = m + 1 \) where \( m = 0 \), thus \( n+1 = (m+1)+1 = m+2 \, \) and \( n+2 = (m+1)+2 = m+3 \). The new series is \( \sum_{m=0}^{\infty} \frac{1}{(m+2)(m+3)} \).
04
Change of Index for (a) to Start at \(n=5\)
For \( \sum_{n=1}^{\infty} \frac{1}{(n+1)(n+2)} \), we add 4 to the index: Substitute \( n = m - 4 \) where \( m = 5 \), thus \( n+1 = (m-4)+1 = m-3 \, \) and \( n+2 = (m-4)+2 = m-2 \). The new series is \( \sum_{m=5}^{\infty} \frac{1}{(m-3)(m-2)} \).
05
Change of Index for (b) to Start at \(n=-2\)
For \( \sum_{n=1}^{\infty} \frac{1}{(n+3)(n+4)} \), we subtract 3 from the index: Substitute \( n = m + 3 \) where \( m = -2 \), thus \( n+3 = (m+3)+3 = m+6 \, \) and \( n+4 = (m+3)+4 = m+7 \). The new series is \( \sum_{m=-2}^{\infty} \frac{1}{(m+6)(m+7)} \).
06
Change of Index for (b) to Start at \(n=0\)
For \( \sum_{n=1}^{\infty} \frac{1}{(n+3)(n+4)} \), we subtract 1 from the index: Substitute \( n = m + 1 \) where \( m = 0 \), thus \( n+3 = (m+1)+3 = m+4 \, \) and \( n+4 = (m+1)+4 = m+5 \). The new series is \( \sum_{m=0}^{\infty} \frac{1}{(m+4)(m+5)} \).
07
Change of Index for (b) to Start at \(n=5\)
For \( \sum_{n=1}^{\infty} \frac{1}{(n+3)(n+4)} \), add 4 to the index: Substitute \( n = m - 4 \) where \( m = 5 \), thus \( n+3 = (m-4)+3 = m-1 \, \) and \( n+4 = (m-4)+4 = m \). The new series is \( \sum_{m=5}^{\infty} \frac{1}{(m-1)m} \).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Index Shifting
Index shifting is a technique used in mathematics when dealing with series. The goal is to change the starting point of a series without altering its overall value. Imagine you have a sequence and you want it to start from a different place. Index shifting allows you to do this by adjusting the indices. This change usually involves adding or subtracting a constant to all indices in the series.
This might sound tricky, but it really boils down to substituting a new variable for the index, changing it to make calculations more manageable.
This might sound tricky, but it really boils down to substituting a new variable for the index, changing it to make calculations more manageable.
- This is helpful if you want to match a series to a different parameter or problem setup.
- It's like moving a camera to view the same scene from another angle.
- Mathematically, this involves operations like \( n = m + c \), where \( c \) is a constant, and \( m \) is the new index representing \( n \).
Infinite Series
An infinite series is a sum of infinitely many numbers in a sequence. Unlike a finite series, which ends at a certain point, an infinite series keeps going on and on. The most iconic representation of an infinite series is the summation sign followed by a function, \( \sum \), indexed over the infinite set.
- Infinite series can converge or diverge. Convergence means the series approaches a specific value, while divergence means it does not.
- An example of a converging series is \( \sum_{n=1}^{\infty} \frac{1}{n^2} \), while an example of a diverging series is \( \sum_{n=1}^{\infty} \frac{1}{n} \).
- The behavior of an infinite series often gives insight into fundamental mathematical principles.
- Understanding whether a series converges or diverges is crucial for further mathematical usages like calculus and analysis.
Mathematical Series
A mathematical series is a sum of the terms of a sequence. You can think of it as listing numbers (the terms) and then deciding to add them together. Unlike a sequence where we list numbers individually, series emphasizes the total result of adding them up.
- Simple examples are arithmetic and geometric series. Arithmetic series add a constant value each step, while geometric series multiply by a constant, like \( a, ar, ar^2, \ldots \).
- A series can be finite or infinite. Finite series have an ending point, whereas infinite series do not.
- Mathematical series highlight patterns and dimensions within numbers.
- The notation \( \sum \) (sigma) represents the summation of a series' terms.
- Every series has its own set of rules or formulas, depending on its type, to calculate the sum efficiently without listing all terms.