Re: Bokurano Daibouken 3 : secondary topic
They're the triangular numbers.
The first triangular number is 1. The second is 1+2, so 3. The third is 3+3, 6. 6+4 = 10. 10+5 = 15. Etc.
You can also work them out with the formula f(n) = n(n+1)/2 where n is the nth term in the sequence.
Therefore 15 is the fifth triangular number, 55 is the 10th, etc. I agree its an unfair puzzle.
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