GEOMETRIC SEQUENCES. These are sequences where the ratio of successive terms of a sequence is always the same number. This number is called the common ratio . 1, 2, 4, 8, 16 . . . r = 2.
These are sequences where the ratio of successive terms of a sequence is always the same number. This number is called the common ratio.
r = 2
Notice in this sequence that if we find the ratio of any term to the term before it (divide them) we always get 2. 2 is then called the common ratio and is denoted with the letter r.
To get to the next term in the sequence we would multiply by 2 so a recursive formula for this sequence is:
r = 2
a = 1
1, 2, 4, 8, 16 . . .
Each time you want another term in the sequence you’d multiply by r. This would mean the second term was the first term times r. The third term is the first term multiplied by r multiplied by r (r squared). The fourth term is the first term multiplied by r multiplied by r multiplied by r (r cubed). So you can see to get the nth term we’d take the first term and multiply r (n - 1) times.
Try this to get the 5th term.
Let’s look at a formula for a geometric sequence and see what it tells us.
you can see what the common ratio will be in the formula
This factor gets us started in the right place. With n = 1 we’d get -2 for the first term
Subbing in the set of positive integers we get:
What is the common ratio?
-2, -6, -18, -54 …
r = 3
3n-1 would generate the powers of 3. With the - 2 in front, the first term would be -2(30) =- 2. What would you do if you wanted the sequence -4, -12, -36, -108, . . .?
If we use 4n-1 we will generate a sequence whose common ratio is 4, but this sequence starts at 1 (put 1 in for n to get first term to see this). We want ours to start at -2. We then need the “compensating factor”. We need to multiply by -2.
Check it out by putting in the first few positive integers and verifying that it generates our sequence.
Sure enough---it starts at -2 and has a common ratio of 4
-2, -8, -32, -128, . . .
To find the common ratio, take any term and divide it by the term in front
If we want to add n terms in a geometric sequence, we use the formula below:
number of terms
sum of n terms
Find the sum:
4 + 12 + 36 + 108 + . . . + 4(3)14
Let’s look at this on the number line
Each time we add another term we’d be going half the distance left. As n the sum 1.
If the common ratio was not a fraction between -1 and 1, then the sequence would keep getting larger and larger and would as n . If the common ratio is a fraction between -1 and 1, the sum asn is as follows:
Let’s try this for the previous sequence:
I wish to thank Shawna Haider from Salt Lake Community College, Utah USA for her hard work in creating this PowerPoint.
Shawna has kindly given permission for this resource to be downloaded from www.mathxtc.com and for it to be modified to suit the Western Australian Mathematics Curriculum.
Head of Mathematics
St Stephen’s School – Carramar