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This is the sequence of triangular numbers. You draw one dot. Then you draw two dots in the line below - one to the left and one to the right of the first dot. You now have a triangle, of three dots. Then you draw three dots in the next line and you get a triangle with 6 dots. Next, four more dots in the next line giving a triangle with 10 dots. The name is easy to understand if you can visualise or even actually draw these dots. It is difficult to demonstrate through an ordinary word processing package.

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Q: What is the name of this sequence 1 3 6 10 15 21?

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Assuming it continues 15, 21, 28, ... then it is the triangular numbers.

28

35

The next number in the sequence 3-15-6-30-21 is -207.

21

5/7 - 5/21 = 15/21 - 5/21 = 10/215/7 - 5/21 = 15/21 - 5/21 = 10/215/7 - 5/21 = 15/21 - 5/21 = 10/215/7 - 5/21 = 15/21 - 5/21 = 10/21

The GCF of 10, 15, and 21 is 1.

Yes it is. In sequence from 1, the first 10 triangular numbers are... 1, 3, 6, 10, 15, 21, 28, 36, 45 & 55.

I'm guessing your sequence is 1, 3, 6, 10, 15, ... In which case it continues: 21, 28, 36, 45, 55, 66, ... (These are the triangular numbers.)

They are 6, 10, 14, 15, 21, 22.They are 6, 10, 14, 15, 21, 22.They are 6, 10, 14, 15, 21, 22.They are 6, 10, 14, 15, 21, 22.

26 As far as I can tell the sequence is +10,-5,+10,-5.

Fibonacci Sequence

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