Problem 19 - Algebra and Trigonometry

Question

17-22 Find the first five terms of the sequence and determine if it is arithmetic. If is arithmetic, find the common difference and express the \(n\)th term of the sequence in the standard form \(a_{n}=a+(n-1) d .\) $$a_{n}=\frac{1}{1+2 n}$$

Step-by-Step Solution

Answer

The sequence is not arithmetic.

Step 1: Understand the sequence

The sequence is given by the formula \(a_{n}=\frac{1}{1+2n}\). This formula tells us how each term in the sequence is calculated based on its position \(n\).

Step 2: Calculate the first five terms

To find the first five terms, substitute \(n=1, 2, 3, 4, 5\) into the sequence formula:- For \(n=1\), \(a_1 = \frac{1}{1+2(1)} = \frac{1}{3}\)- For \(n=2\), \(a_2 = \frac{1}{1+2(2)} = \frac{1}{5}\)- For \(n=3\), \(a_3 = \frac{1}{1+2(3)} = \frac{1}{7}\)- For \(n=4\), \(a_4 = \frac{1}{1+2(4)} = \frac{1}{9}\)- For \(n=5\), \(a_5 = \frac{1}{1+2(5)} = \frac{1}{11}\).

Step 3: Determine if the sequence is arithmetic

An arithmetic sequence has a constant difference between consecutive terms. Calculate the differences:- \(a_2 - a_1 = \frac{1}{5} - \frac{1}{3} = -\frac{2}{15}\)- \(a_3 - a_2 = \frac{1}{7} - \frac{1}{5} = -\frac{2}{35}\)- \(a_4 - a_3 = \frac{1}{9} - \frac{1}{7} = -\frac{2}{63}\)- \(a_5 - a_4 = \frac{1}{11} - \frac{1}{9} = -\frac{2}{99}\)The differences are not constant, so this is not an arithmetic sequence.

Key Concepts

Sequences and Series

In mathematics, sequences and series are fundamental concepts that help us understand patterns of numbers. A sequence is simply an ordered list of numbers, where each number is called a term. These sequences can follow a specific pattern or rule that dictates how the terms are determined. For example, the sequence given by the formula \(a_{n} = \frac{1}{1 + 2n}\) tells us that the nth term is calculated using this mathematical expression.

This particular sequence results in terms that gradually decrease as \(n\) increases. Recognizing the pattern in a sequence can help determine the type of sequence it is, such as arithmetic, geometric, or neither. On the other hand, a series involves summing the terms of a sequence. It's important to differentiate between the two concepts as they are used in various applications of mathematics, such as calculus and algebra. Hence, comprehending sequences is a crucial step before diving deeper into series, which builds upon this fundamental understanding.

Common Difference

A common difference is a unique feature of an arithmetic sequence. An arithmetic sequence is defined by the constant difference between consecutive terms. This difference is called the common difference and is denoted by \(d\).

For example, in a sequence like 2, 4, 6, 8, the common difference is \(2\), because each term increases by \(2\) as you move from one to the next. However, not every sequence with increasing or decreasing terms is arithmetic. In the exercise we reviewed, the sequence given by \(a_{n} = \frac{1}{1 + 2n}\) showed varying differences between terms.

Upon calculating, we found differences such as \(-\frac{2}{15}\), \(-\frac{2}{35}\), and others, which are not constant. This irregularity indicates that the sequence is not arithmetic. Identifying whether a sequence has a common difference is key in classifying it and further allows for determining the formula for the nth term when applicable.

Formula for nth Term

The formula for the nth term in a sequence allows you to compute any term's value without manually listing out every preceding term. For arithmetic sequences, this formula is given as \(a_n = a + (n-1) \cdot d\), where \(a\) is the first term and \(d\) is the common difference.

Using this formula simplifies the process of finding specific terms, especially when the sequence is arithmetic. However, it's important to accurately identify the type of sequence before applying this formula.

In our specific exercise, the sequence \(a_{n} = \frac{1}{1+2n}\) did not have a common difference, meaning it was not arithmetic. Therefore, the standard nth term formula for arithmetic sequences could not be applied here.

For non-arithmetic sequences, other unique formulas or defining rules are used to determine their terms. Understanding how to use the formula for the nth term effectively saves time and streamlines the problem-solving process in cases where it is applicable.