Difference between revisions of "Arithmetic sequence"
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− | ==See Also== | + | In [[algebra]], an '''arithmetic sequence''', sometimes called an '''arithmetic progression''', is a [[sequence]] of numbers such that the difference between any two consecutive terms is constant. This constant is called the '''common difference''' of the sequence. |
− | [[ | + | |
+ | For example, <math>1, 2, 3, 4</math> is an arithmetic sequence with common difference <math>1</math> and <math>99, 91, 83, 75</math> is an arithmetic sequence with common difference <math>-8</math>; however, <math>7, 0, 7, 14</math> and <math>4, 12, 36, 108, \ldots</math> are not arithmetic sequences, as the difference between consecutive terms varies. | ||
+ | |||
+ | More formally, the sequence <math>a_1, a_2, \ldots , a_n</math> is an arithmetic progression if and only if <math>a_2 - a_1 = a_3 - a_2 = \cdots = a_n - a_{n-1}</math>. A similar definition holds for infinite arithmetic sequences. It appears most frequently in its three-term form: namely, that constants <math>a</math>, <math>b</math>, and <math>c</math> are in arithmetic progression if and only if <math>b - a = c - b</math>. | ||
+ | |||
+ | == Properties == | ||
+ | Because each term is a common distance from the one before it, every term of an arithmetic sequence can be expressed as the sum of the first term and a multiple of the common difference. Let <math>a_1</math> be the first term, <math>a_n</math> be the <math>n</math>th term, and <math>d</math> be the common difference of any arithmetic sequence; then, <math>a_n = a_1 + (n-1)d</math>. | ||
+ | |||
+ | A common lemma is that given the <math>n</math>th term <math>x</math> and <math>m</math>th term <math>y</math> of an arithmetic sequence, the common difference is equal to <math>\frac{y-x}{m-n}</math>. | ||
+ | |||
+ | '''Proof''': Let the sequence have first term <math>a_1</math> and common difference <math>d</math>. Then using the above result, <cmath>\frac{y-x}{m-n} = \frac{(a_1 + (m - 1)d) - (a_1 + (n-1)d)}{m-n} = \frac{dm - dn}{m-n} = d,</cmath> as desired. <math>\square</math> | ||
+ | |||
+ | Another common lemma is that a sequence is in arithmetic progression if and only if <math>a_n</math> is the [[arithmetic mean]] of <math>a_{n-1}</math> and <math>a_{n+1}</math> for any consecutive terms <math>a_{n-1}, a_n, a_{n+1}</math>. In symbols, <math>a_n = \frac{a_{n-1} + a_{n+1}}{2}</math>. This is mostly used to perform substitutions, though it occasionally serves as a definition of arithmetic sequences. | ||
+ | |||
+ | == Sum == | ||
+ | An '''arithmetic series''' is the sum of all the terms of an arithmetic sequence. All infinite arithmetic series diverge. As for finite series, there are two primary formulas used to compute their value. | ||
+ | |||
+ | The first is that if an arithmetic series has first term <math>a_1</math>, last term <math>a_n</math>, and <math>n</math> total terms, then its value is equal to <math>\frac{n(a_1 + a_n)}{2}</math>. | ||
+ | |||
+ | '''Proof''': Let the series be equal to <math>S</math>, and let its common difference be <math>d</math>. Then, we can write <math>S</math> in two ways: <cmath>S = a_1 + (a_1 + d) + \cdots + (a_1 + (n-1)d)</cmath> <cmath>S = a_n + (a_n - d) + \cdots + (a_n - (n-1)d.</cmath> Adding these two equations cancels all terms involving <math>d</math>; <cmath>2S = (a_1 + a_n) + (a_1 + a_n) + \cdots + (a_1 + a_n) = n(a_1 + a_n),</cmath> and so <math>S = \frac{n(a_1 + a_n)}{2}</math>, as required. <math>\square</math> | ||
+ | |||
+ | The second is that if an arithmetic series has first term <math>a_1</math>, common difference <math>d</math>, and <math>n</math> terms, it has value <math>\frac{n(2a_1 + (n-1)d)}{2}</math>. | ||
+ | |||
+ | '''Proof''': The final term has value <math>a_1 + (n-1)d</math>. Then by the above formula, the series has value <cmath>\frac{n(a_1 + (a_1 + (n-1)d)}{2} = \frac{n(2a_1 + (n-1)d)}{2}.</cmath> This completes the proof. <math>\square</math> | ||
+ | |||
+ | == Problems == | ||
+ | Here are some problems with solutions that utilize arithmetic sequences and series. | ||
+ | |||
+ | === Introductory problems === | ||
+ | * [[2005_AMC_10A_Problems/Problem_17 | 2005 AMC 10A Problem 17]] | ||
+ | * [[2006_AMC_10A_Problems/Problem_19 | 2006 AMC 10A Problem 19]] | ||
+ | * [[2012 AIME I Problems/Problem 2]] | ||
+ | * [[2004 AMC 10B Problems/Problem 10]] | ||
+ | * [[2006_AMC_10A_Problems/Problem_9 | 2006 AMC 10A, Problem 9]] | ||
+ | * [[2006 AMC 12A Problems/Problem 12 | 2006 AMC 12A, Problem 12]] | ||
+ | |||
+ | === Intermediate problems === | ||
+ | * [[2003 AIME I Problems/Problem 2|2003 AIME I, Problem 2]] | ||
+ | * Find the roots of the polynomial <math>x^5-5x^4-35x^3+ax^2+bx+c</math>, given that the roots form an arithmetic progression. | ||
+ | |||
+ | == See Also == | ||
+ | * [[Geometric sequence]] | ||
+ | * [[Harmonic sequence]] | ||
+ | * [[Sequence]] | ||
+ | * [[Series]] | ||
+ | |||
+ | [[Category:Algebra]] [[Category:Sequences and series]] [[Category:Definition]] |
Latest revision as of 02:00, 7 March 2024
In algebra, an arithmetic sequence, sometimes called an arithmetic progression, is a sequence of numbers such that the difference between any two consecutive terms is constant. This constant is called the common difference of the sequence.
For example, is an arithmetic sequence with common difference
and
is an arithmetic sequence with common difference
; however,
and
are not arithmetic sequences, as the difference between consecutive terms varies.
More formally, the sequence is an arithmetic progression if and only if
. A similar definition holds for infinite arithmetic sequences. It appears most frequently in its three-term form: namely, that constants
,
, and
are in arithmetic progression if and only if
.
Contents
Properties
Because each term is a common distance from the one before it, every term of an arithmetic sequence can be expressed as the sum of the first term and a multiple of the common difference. Let be the first term,
be the
th term, and
be the common difference of any arithmetic sequence; then,
.
A common lemma is that given the th term
and
th term
of an arithmetic sequence, the common difference is equal to
.
Proof: Let the sequence have first term and common difference
. Then using the above result,
as desired.
Another common lemma is that a sequence is in arithmetic progression if and only if is the arithmetic mean of
and
for any consecutive terms
. In symbols,
. This is mostly used to perform substitutions, though it occasionally serves as a definition of arithmetic sequences.
Sum
An arithmetic series is the sum of all the terms of an arithmetic sequence. All infinite arithmetic series diverge. As for finite series, there are two primary formulas used to compute their value.
The first is that if an arithmetic series has first term , last term
, and
total terms, then its value is equal to
.
Proof: Let the series be equal to , and let its common difference be
. Then, we can write
in two ways:
Adding these two equations cancels all terms involving
;
and so
, as required.
The second is that if an arithmetic series has first term , common difference
, and
terms, it has value
.
Proof: The final term has value . Then by the above formula, the series has value
This completes the proof.
Problems
Here are some problems with solutions that utilize arithmetic sequences and series.
Introductory problems
- 2005 AMC 10A Problem 17
- 2006 AMC 10A Problem 19
- 2012 AIME I Problems/Problem 2
- 2004 AMC 10B Problems/Problem 10
- 2006 AMC 10A, Problem 9
- 2006 AMC 12A, Problem 12
Intermediate problems
- 2003 AIME I, Problem 2
- Find the roots of the polynomial
, given that the roots form an arithmetic progression.