Question

write a rule for the nth term of the sequence. Then fi nd a20. \(51,48,45,42, \ldots\)

Short answer

The rule for the nth term of the sequence is \(a_n = 54 - 3n\). The 20th term of the sequence is -6.

Step-by-step solution

Identify first term (a1) and common difference (d)

Looking at the sequence \(51,48,45,42, \ldots\), it is clear that the first term a1 is 51. The sequence is decreasing by 3 each time, so the common difference (d) is -3.

Construct the nth term formula

The general rule for the nth term of an arithmetic sequence is \(a_n = a_1 + (n-1) * d\). To fit our sequence, this will be \(a_n = 51 + (n-1) * -3\). Simplifying this gives \(a_n = 54 - 3n\). This formula allows us to find any term in the sequence.

Find the 20th term (a20)

To find the 20th term, substitute n = 20 into the term formula: \(a_{20} = 54 - 3 * 20 = 54 - 60 = -6\).

Key concepts

nth term of a sequence

Understanding the nth term of a sequence is paramount for cracking the pattern of an arithmetic sequence. Each number in the sequence is termed as a 'term', and the position of that term in the sequence is called its 'index'. For example, in the sequence \(51, 48, 45, 42, \dots\), the number 51 is the first term (also designated as \(a_1\)), and the number 45 is the third term, indicating its index.

The nth term is a formula that helps you find any term in the sequence without having to list all of the terms before it. This becomes especially handy with longer sequences. The general formula is \(a_n = a_1 + (n-1) * d\), where:

• \(a_n\) represents the nth term we're trying to find,
• \(a_1\) is the first term in the sequence,
• \(n\) is the index of the term,
• and \(d\) is the common difference between the terms.

Using this, you can immediately find that the 20th term (\(a_{20}\)) in our given sequence is -6, without manually subtracting 3 twenty times.

arithmetic sequence formula

The arithmetic sequence formula is a powerful tool that defines the linear relationship between consecutive terms in a sequence. An arithmetic sequence is one where the difference between successive terms is constant, called the common difference (\(d\)).

The standardized arithmetic sequence formula is expressed as \(a_n = a_1 + (n-1) * d\). What this formula does is it starts with the first term (\(a_1\)), then adds the common difference (\(d\)) enough times to reach the nth term. This repeated addition is equivalent to multiplication, whereby (\(n-1\)) represents how many times the common difference is added to the first term.

Using the Formula

For our sequence \(51, 48, 45, 42, \dots\), we can write the formula as \(a_n = 51 + (n-1) * (-3)\). This formula simplifies processes and makes it easy not just to find any given term in the sequence, but also to understand the sequence's behavior at large.

common difference in sequences

The common difference in arithmetic sequences is the engine that drives the series of numbers. It's the constant value added to each term to get the next term in the sequence. For the sequence we are considering, each number is followed by the next number which is 3 less than itself, indicating a common difference of -3.

Identifying the common difference is straightforward:

• Subtract the first term from the second term (\(a_2 - a_1\)), or any term from the term immediately following it (\(a_{n+1} - a_n\)),
• The result is the common difference (\(d\)).

Understanding the common difference is crucial as it defines the unique characteristic of the sequence. It doesn't just show the interval between the numbers; it also lays the foundation for finding the general formula of the sequence. To put it simply, without the common difference, we cannot accurately predict the future terms of the sequence or understand its growth or decay patterns.