Geometric Sequences and Series
This page will teach you about geometric sequences and series. These are the sections within this page:
Sequences of numbers that follow a pattern of multiplying a fixed number from one term to the next are called geometric sequences. The following sequences are geometric sequences:
Sequence A:   1 , 2 , 4 , 8 , 16 , ...
For sequence A, if we multiply by 2 to the first number we will get the second number. This works for any pair of consecutive numbers. The second number times 2 is the third number: 2 × 2 = 4, and so on.
For sequence B, if we multiply by 6 to the first number we will get the second number. This also works for any pair of consecutive numbers. The third number times 6 is the fourth number: 0.36 × 6 = 2.16, which will work throughout the entire sequence.
Sequence C is a little different because it seems that we are dividing; yet to stay consistent with the theme of geometric sequences, we must think in terms of multiplication. We need to multiply by -1/2 to the first number to get the second number. This too works for any pair of consecutive numbers. The fourth number times -1/2 is the fifth number: -2 × -1/2 = 1.
Because these sequences behave according to this simple rule of multiplying a constant number to one term to get to another, they are called geometric sequences. So that we can examine these sequences to greater depth, we must know that the fixed numbers that bind each sequence together are called the common ratios. Mathematicians use the letter r when referring to these types of sequences.
Mathematicians also refer to generic sequences using the letter a along with subscripts that correspond to the term numbers as follows:
This means that if we refer to the tenth term of a certain sequence, we will label it a10. a14 is the 14th term. This notation is necessary for calculating nth terms, or an, of sequences.
The r-value can be calculated by dividing any two consecutive terms in a geometric sequence. The formula for calculating r is...
...where n is any positive integer greater than 1.
In order for us to know how to obtain terms that are far down these lists of numbers, we need to develop a formula that can be used to calculate these terms. If we were to try and find the 20th term, or worse the 2000th term, it would take a long time if we were to simply multiply a number -- one at a time -- to find our terms.
If we had to find the 400th term of sequence A above, we would undertake a tedious task had we decided to multiply by two each step of the way all the way to the 400th term. Luckily, there is a way to arrive at the 400th term without the need for calculating terms 1 through 399.
The formula for the general term for each geometric sequence is...
uizmaster: Finding Formula for General Term
It may be necessary to calculate the number of terms in a certain geometric sequence. To do so, we would need to know two things.
We would need to know a few terms so that we could calculate the common ratio and ultimately the formula for the general term. We would also need to know the last number in the sequence.
Once we know the formula for the general term in a sequence and the last term, the procedure is relatively uncomplicated, but the technique needed to complete the procedure is complicated.
Set the last term equal to the formula for the general term. Since the formula uses the variable n to calculate terms, we can also use it to determine the term number for any given term.
If we again look at sequence A above, let's use the formula that was found to calculate term values, an = a1rn - 1. If we knew that 256 was a number in the sequence (1, 2, 4, 8, 16, ..., 256 ) we would set the number 256 equal to the formula an = a1rn - 1 and get 256 = 2n - 1. Solving this equation using proper techniques requires the use of logarithms and would yield n = 9. [We could use the method of guessing and checking to arrive at the same value.] This means that there are 9 terms in the mentioned sequence and that the 9th term, a9, is equal to 256.
Let's look at a portion of sequence C. If the sequence went from 16 to -1/8, we would have: 16, -8, 4, ..., -1/8. We would use the formula for the general term...
...and set it equal to the last term, -1/8. We would get -1/8 = (16)(-1/2)n - 1. Solving this equation [with the use of either logarithms or the method of guessing and checking] allows us to arrive at n = 8. This means that there are 8 terms in the sequence and that a8 = -1/8.
Given our generic arithmetic sequence a1, a2, a3, a4, ..., an, we can look at it as a series: a1 + a2 + a3 + a4 + ... + an. As we can see, the only difference between a sequence and a series is that a sequence is a list of numbers and a series is a sum of numbers.
There exists a formula that can add a finite list of these numbers and a formula for an infinite list of these numbers. Here are the formulas...
...where Sn is the sum of the first n numbers, a1 is the first number in the sequence, r is the common ratio of the sequence, and -1 < r < 1 for infinite series. Let's use examples to investigate both formulas.
Formula One: Finite Sum
Formula Two: Infinite Sum
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