1, 3 and 6
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no No it is not. The first 5 triangular numbers are: 1, 3, 6, 10, 15
The first two triangle numbers are 3 and 6. [Unless you start with 1 as the first.]
The first twelve triangular numbers are: 1 3 6 10 15 21 28 36 45 55 66 78
The first six triangular numbers are 1, 3, 6, 10, 15 and 21.
1, 3, 6, 10, 15