1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 104, ...
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The answer would be 15 and 21. The triangular numbers are 1,3,6,10,15,21,28,36,45,55,66,78. 15 plus 21 is 36 which is on the list. 21 minus 15 is 6 and that is on the list as well.
1, 3, 6, 10, 15, 21, 28, 36
Triangle numbers or triangular numbers are those numbers that can form an equilateral triangle when counting the objects. The first five triangular numbers are: 1, 3, 6, 10, 15.
6 and 10 are triangular numbers that make 16.
the answer is of course 12