Geometric sequence

Revision as of 09:54, 23 June 2006 by JBL (talk | contribs) (Summing a Geometric Sequence)

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

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

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

Infinate Geometric Sequences

An infinate geometric sequence is a geometric sequence with an infinate number of terms. These sequences can have sums, sometimes called limits, if $|r|<1$.

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

$S = \frac{a_1}{1-r}$

Again, $a_1$ is the first term in the sequence, and $r$ is the common ratio.

See Also

Arithmetic Sequences