What is a sequence?
A sequence is a series of numbers that have an order and have a specific number pattern in the order. Getting from one number to the next can be thought of as a step. A simple pattern is formed when the same number is added or subtracted at each step. Let's take a look at the examples below:
This is an increasing sequence, where $2$2 is added at every step
This is a decreasing sequence, where $3$3 is subtracted at every step
To find the next number that follows in a sequence, it's as simple as finding the pattern and applying to the last number. For example, the next number in the decreasing sequence above would be $5-3=2$5−3=2.
Practice questions
Question 1
Find the next number in the sequence:
$7$7, $9$9, $11$11, $13$13, $\editable{}$
Describing sequences
In mathematics, a sequence is often given as a list of numbers, each separated by commas. Each of the separate numbers in a sequence can be called a term. The terms in a sequence are referred to using subscript numbers. For example the initial term is commonly labelled, $t_0$t0, and the first term, $t_1$t1, and the second term, $t_2$t2 and so on. In general, any $n$nth number term in a sequence can be referred to as $t_n$tn and any previous term would be $t_{n-1}$tn−1 and the subsequent term would be $t_{n+1}$tn+1
Note that any letter could be used to refer to the terms, not just $t$t.
If the sequence ends, it is known as a finite sequence. $-3,5,13,21$−3,5,13,21 and $1,10,100,1000,10000$1,10,100,1000,10000 are examples of finite sequences. An infinite sequence is a sequence with infinite terms, in other words a sequence that never ends. $1,1/2,1/3,1/4,1/5,1/6,...$1,1/2,1/3,1/4,1/5,1/6,... is an example of an infinite sequence, where one keeps being added to the denominator to create the next term. The "dot dot dot" $...$... at the end of the sequence means that the number pattern in the sequence continues indefinitely, making this an infinite sequence versus a finite sequence.
When a number pattern is detectable in a progression, a generating rule can often be established and then used to determine a term in the sequence. Mathematicians can sometimes develop explicit generating rules that allow the calculation of any particular term in the sequence. For example the rule $t_n=\frac{1}{n}$tn=1n means that the $n$nth term is the reciprocal of $n$n. This is an example where $n=1,2,3,...$n=1,2,3,... , so $t_1$t1 would be the first term and not $t_0$t0 , as $n=0$n=0 cannot be substituted into this generating rule. So the first term becomes $t_1=\frac{1}{1}=1$t1=11=1 and the second term $t_2=\frac{1}{2}$t2=12 etc., so that the sequence becomes $1,1/2,1/3,1/4,1/5,1/6,...$1,1/2,1/3,1/4,1/5,1/6,... and so on.
There are some sequences that appear to have no pattern at all and therefore is difficult or impossible to find an explicit generating rule. But they can nevertheless have a certain logical way of building. For example, the sequence $3,1,4,1,5,9,...$3,1,4,1,5,9,... separating the digits of $\pi$π exhibits no discernible pattern and continues to do so indefinitely.
Practice questions
QUESTION 2
State the first five terms of the sequence $a_n=3n-3$an=3n−3, starting from $n=1$n=1.
Write all five terms on the same line separated by a comma.
Question 3
State the first five terms of the sequence $a_n=2^{n+1}$an=2n+1.
Write all five terms on the same line separated by a comma.
Using tables and graphs to describe a sequence
Below is a drawing of a simple pattern:
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| Flowers $n$n | $1$1 | $2$2 | $3$3 | $4$4 |
| Petals $a_n$an | $5$5 | $10$10 | $15$15 | $20$20 |
In this pattern, $n$n represents the step number in the sequence (that is, the number of flowers) and $a_n$an represents the total number of petals at that step.
Notice that the number of petals are increasing by $5$5 each time - in particular, the value of $a_n$an is always equal to $5$5 times the value of $n$n. Therefore, the generating rule for this sequence must be $a_n=5n$an=5n, where $a_1=5$a1=5 represents the first term.
This rule can now be used to predict future results. For example, to calculate the total number of petals when there are $10$10 flowers present, substitute $n=10$n=10 into the rule to find $a_{10}=5\times10=50$a10=5×10=50 petals. So even though there were only $1,2,3$1,2,3 and $4$4 flowers present in the sequence above, the rule has determined that there would be $50$50 petals visible when there are $10$10 flowers present.
A graph can also be used to represent a sequence. Consider the number pattern above, the number of petals $a_n$an versus the number of flowers $n$n is shown in the graph below.
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Practice questions
Question 4
Matches were used to make the pattern attached:
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Complete the table:
Number of triangles ($t$t) $1$1 $2$2 $3$3 $5$5 $10$10 $20$20 Number of matches ($m$m) $\editable{}$ $\editable{}$ $\editable{}$ $\editable{}$ $\editable{}$ $\editable{}$ Write a formula that describes the relationship between the number of matches, $m$m, and the number of triangles, $t$t.
How many matches are required to make $77$77 triangles using this pattern?