SOLUTION: 1,3,6,10.15, ... , n?

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Question 891699: 1,3,6,10.15, ... , n?
Found 2 solutions by robertb, richwmiller:
Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
n+=+%28k%2A%28k%2B1%29%29%2F2, where k is the place of the number in the sequence. E.g., the 5th term is n = (5*6)/2 = 15, etc...

Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
n-----1dif----2dif-----3dif---4difference
1 2 1 0 0
3 3 1 0 0
6 4 1 0
10 5 1
15 6
21
The sixth term could be 21