Triangular Numbers
Algebra
Level
pending
What is the sum of the first 15 triangular numbers?
The answer is 560.
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1 solution
The first 1 5 triangular numbers are 1 , 3 , 6 , 1 0 , 1 5 , 2 1 , 2 8 , 3 6 , 4 5 , 5 5 , 6 6 , 7 8 , 9 1 , 1 0 5 , 1 2 0 so it think the sum is 6 8 0 . Not sure how the sum is 5 6 0 . Correct me if I am mistaken.