A sequence is a list of numbers separated by commas, that usually follow a
special rule:
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1.

5,8,11,14,17,21,...

The rule here is "Start with 5 and add 3 each time to get the next number in
the sequence". That's called an "arithmetic sequence".
It's pronounced "ah-rith-MET-ick"
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2.

3,6,12,24,48,96,...

The rule here is "Start with 3 and MULTIPLY by 2 each time to get the next
number in the sequence". That's called an "geometric sequence".

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3.  Here is the Fibonacci sequence:

1,1,2,3,5,8,13,21,...

The rule here is "Start with 1,1" then add the two previous numbers together
to get the next number in the sequence.  

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4. The sequence of prime numbers:

2,3,5,7,11,13,17,23,...

The rule here is to start with 2, and list the next higher whole number that 
can be divided evenly ONLY by itself and 1.

Edwin