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Difference between revisions of "Geometric sequence"

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For instance, the series <math>1 + \frac12 + \frac14 + \frac18 + \cdots</math>, sums to 2.  The general formula for the sum of such a sequence is:
 
For instance, the series <math>1 + \frac12 + \frac14 + \frac18 + \cdots</math>, sums to 2.  The general formula for the sum of such a sequence is:
  
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<math>S = \frac{a_0}{1-r}</math>
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<math>S = \frac{a_0}{1-r}</math>. Proof: Let the sequence be <math>S=a+ar+ar^2+ar^3+\ldots</math>. Let that equation be (1). We can multiply (1) by r to get: <math>S \cdot r=ar+ar^2+ar^3+\ldots</math>. Let this be (2). We subtract these equations to get: <math> S-S\cdot r=a</math>. There is only one term on the RHS because the rest of the terms cancel out after subtraction. Finally, we can factor and divide, to get <math>S(1-r)=a</math>, thus <math>S=\frac{a}{1-r}</math>. This method of multiplying the sequence and subtracting equations is a frequently used method to evaluate sequences.
  
 
Again, <math>a_0</math> is the first term in the sequence, and <math>r</math> is the common ratio.
 
Again, <math>a_0</math> is the first term in the sequence, and <math>r</math> is the common ratio.

Revision as of 10:05, 23 June 2006

Definition

A geometric sequence is a sequence of numbers in which each term is a fixed multiple of the previous term. For example: 1, 2, 4, 8, 16, 32, ... is a geometric sequence because each term is twice the previous term. In this case, 2 is called the common ratio of the sequence. More formally, a geometric sequence may be defined recursively by:

$a_n = r\cdot a_{n-1}, n \geq 1$

with a fixed $a_0$ and common ratio $r$. Using this definition, the $n$th term has the closed-form:

$\displaystyle a_n = a_0\cdot r^n$

Summing a Geometric Sequence

The sum of the first $n$ terms of a geometric sequence is given by

$S_n = a_0 + a_1 + \ldots + a_{n - 1} = a_0\cdot\frac{r^n-1}{r-1}$

where $a_0$ is the first term in the sequence, and $r$ is the common ratio.

Infinite Geometric Sequences

An infinite geometric sequence is a geometric sequence with an infinite number of terms. If the common ratio is small, the sum of the terms will approach a fixed limit. In this case, "small" means $|r|<1$. We say that the sum of the terms of this sequence is a convergent sum.

For instance, the series $1 + \frac12 + \frac14 + \frac18 + \cdots$, sums to 2. The general formula for the sum of such a sequence is:

$S = \frac{a_0}{1-r}$. Proof: Let the sequence be $S=a+ar+ar^2+ar^3+\ldots$. Let that equation be (1). We can multiply (1) by r to get: $S \cdot r=ar+ar^2+ar^3+\ldots$. Let this be (2). We subtract these equations to get: $S-S\cdot r=a$. There is only one term on the RHS because the rest of the terms cancel out after subtraction. Finally, we can factor and divide, to get $S(1-r)=a$, thus $S=\frac{a}{1-r}$. This method of multiplying the sequence and subtracting equations is a frequently used method to evaluate sequences.

Again, $a_0$ is the first term in the sequence, and $r$ is the common ratio.

One common instance of summing infinite geometric sequences is the decimal expansion of most rational numbers. For instance, $0.33333\ldots = \frac 3{10} + \frac3{100} + \frac3{1000} + \frac3{10000} + \ldots$ has first term $a_0 = \frac 3{10}$ and common ratio $\frac1{10}$, so the infinite sum has value $S = \frac{\frac3{10}}{1-\frac1{10}} = \frac13$, just as we would have expected.

See Also

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