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.
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the answer is of course 12
31 and 18
53 and 22..
-76 and 76 whose product is -5776.
100.9-75.3