21, 28, 36, 45
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3, 6, 10, 15
Yes, the number 21 is indeed a triangular number. Other triangular numbers are 6,10, and 15.
no No it is not. The first 5 triangular numbers are: 1, 3, 6, 10, 15
First few triangular numbers: 0, 1, 3, 6, 10, 15, 21, 28 Numbers that can be thrown with four dice: 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 Which of these numbers that are triangular: 6, 10, 15, 21 None of these is prime.
Thre are infinitely many solutions. The smalest set with two different triangular number adding to another is 6 + 15 = 21