The triangular numbers are 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, ... each of which is the sum of natural numbers: e.g., 15=5+4+3+2+1. The sums of the triangular numbers are, 1, 4, 10, 20, 35, 56, 84, ... Therefore the sum of the first 3 is 10, the sum of the first 5 is 35, the sum of the first 7 is 84, first 9 165, first 11 286. However, the numbers 15 and 70 don’t match these. 15 is the 5th triangular number, but 70 isn’t a triangular number, but is the sum of 15 and 55 (5th and 10th triangular numbers).