Movavi screen recorder studio 10 1 0 mac tnt. To find a missing number in a Sequence, first we must have a Rule
A Sequence is a set of things (usually numbers) that are in order.
Each number in the sequence is called a term (or sometimes 'element' or 'member'), read Sequences and Series for a more in-depth discussion.
To find a missing number, first find a Rule behind the Sequence.
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Sometimes we can just look at the numbers and see a pattern:
Answer: they are Squares (12=1, 22=4, 32=9, 42=16, .)
Rule: xn = n2
Sequence: 1, 4, 9, 16, 25, 36, 49, .
Did you see how we wrote that rule using 'x' and 'n' ?
xn means 'term number n', so term 3 is written x3
And we can calculate term 3 using:
x3 = 32 = 9
We can use a Rule to find any term. For example, the 25th term can be found by 'plugging in' 25 wherever n is.
x25 = 252 = 625
How about another example:
After 3 and 5 all the rest are the sum of the two numbers before,
That is 3 + 5 = 8, 5 + 8 = 13 etc, which is part of the Fibonacci Sequence:
3, 5, 8, 13, 21, 34, 55, 89, .
Which has this Rule:
Rule: xn = xn-1 + xn-2
Now what does xn-1 mean? It means 'the previous term' as term number n-1 is 1 less than term number n.
And xn-2 means the term before that one.
Let's try that Rule for the 6th term:
x6 = x6-1 + x6-2 Forecast bar 2 7 3.
x6 = x5 + x4
So term 6 equals term 5 plus term 4. We already know term 5 is 21 and term 4 is 13, so:
x6 = 21 + 13 = 34
One of the troubles with finding 'the next number' in a sequence is that mathematics is so powerful we can find more than one Rule that works.
Here are three solutions (there can be more!):
Solution 1: Add 1, then add 2, 3, 4, .
So, 1+1=2, 2+2=4, 4+3=7, 7+4=11, etc.
Rule: xn = n(n-1)/2 + 1
Sequence: 1, 2, 4, 7, 11, 16, 22, .
(That rule looks a bit complicated, but it works)
Solution 2: After 1 and 2, add the two previous numbers, plus 1:
Rule: xn = xn-1 + xn-2 + 1
Sequence: 1, 2, 4, 7, 12, 20, 33, .
Solution 3: After 1, 2 and 4, add the three previous numbers
Rule: xn = xn-1 + xn-2 + xn-3
Sequence: 1, 2, 4, 7, 13, 24, 44, .
So, we have three perfectly reasonable solutions, and they create totally different sequences.
Which is right? They are all right.
And there are other solutions .. it may be a list of the winners' numbers . so the next number could be . anything! |
When in doubt choose the simplest rule that makes sense, but also mention that there are other solutions.
Sometimes it helps to find the differences between each pair of numbers . this can often reveal an underlying pattern.
Launchey 1 4 0 download free. Here is a simple case:
The differences are always 2, so we can guess that '2n' is part of the answer.
Let us try 2n:
The last row shows that we are always wrong by 5, so just add 5 and we are done:
Rule: xn = 2n + 5
OK, we could have worked out '2n+5' by just playing around with the numbers a bit, but we want a systematic way to do it, for when the sequences get more complicated.
In the sequence {1, 2, 4, 7, 11, 16, 22, .} we need to find the differences .
. and then find the differences of those (called second differences), like this:
The second differences in this case are 1.
With second differences we multiply by n22
In our case the difference is 1, so let us try just n22:
n: | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|
Terms (xn): | 1 | 2 | 4 | 7 | 11 |
n22: | 0.5 | 2 | 4.5 | 8 | 12.5 |
Wrong by: | 0.5 | 0 | -0.5 | -1 | -1.5 |
We are close, but seem to be drifting by 0.5, so let us try: n22 − n2
Wrong by 1 now, so let us add 1:
n22 − n2 + 1 | 1 | 2 | 4 | 7 | 11 |
---|---|---|---|---|---|
Wrong by: | 0 | 0 | 0 | 0 | 0 |
We did it!
The formula n22 − n2 + 1 can be simplified to n(n-1)/2 + 1
So by 'trial-and-error' we discovered a rule that works:
Rule: xn = n(n-1)/2 + 1
Sequence: 1, 2, 4, 7, 11, 16, 22, 29, 37, .
Read Sequences and Series to learn about:
And there are also:
And many more!
In truth there are too many types of sequences to mention here, but if there is a special one you would like me to add just let me know.