Maths Physics 
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Sequences
A sequence is  a list of numbers which  follow
  a rule or pattern.
The most simple sequence is
1,2,3,4,5,6,7,8,9, where the numbers go up in ones.
So the pattern or rule for this sequence is add 1 each time
The difference between each number is one
1,3,5,7,9, is an odd number sequence  and 4,6,8,10
In these sequences the difference between each  number is two
You can have a sequence where the differences follow a pattern
1,    2,   4,    7,  11,  16
     Differences                    1    2     3    4     5
In this the difference increases by 1 each time
The next number would be 22 (increase by 6)
When you are asked to continue a sequence or fill in some missing
numbers in a sequence you have to work out the pattern or rule
of the sequence  

1, 4,  9, 16, 25  is a square number sequence  
1, 8, 27, 64, 125 is a cube number sequence
3, 6, 9, 12, is when the pattern number is multiplied by 3 each time
There are all kinds of rules


The nth number
Look at this sequence
 Sequence                  2,   4,   6,   8,   10  .........?
Pattern number (n)      1,   2,   3,   4,   5 ...........n
The numbers in the sequence are twice the pattern number
                          So the nth number is 2n
Look at this sequence
 Sequence                  1,   3,   5,   7,   9  .........?
Pattern number (n)      1,   2,   3,   4,   5 ...........n
The numbers in the sequence are twice the pattern number
less 1
So the nth number is 2n - 1
Look at this sequence
 Sequence                  3,   6,   9, 12, 15  .........?
Pattern number (n)      1,   2,   3,   4,   5 ...........n
The numbers in the sequence are 3 times the pattern number

So the nth number is 3n
Look at this sequence
 Sequence                  5,   8,  11, 14, 17  .........?
Pattern number (n)      1,   2,   3,   4,   5 ...........n
The numbers in the sequence are 3 times the pattern number
+2

So the nth number is 3n + 1
The rule for finding the nth number is to look
at the difference, use this to multiply
by n (e.g. 2n) then look what needs
to be added or subtracted to make it  
fit the sequence
(e.g. 2n + 2)  or (3n - 1)
In a square number sequence the nth number is n2
In a cube number sequence the nth number is n3
You can also add and subtract to square and cube
sequences
e.g. 2, 5, 10, 17  the nth number is n2 + 1
and 0, 3, 8, 15 would be n2 - 1