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numbers of the form n(n+1)/2, seen in triangular packings


a sequence of triangular numbers
as you said 3 as 1st
i.e. n=2 is first then 
n=11 will be 10th

by substituting n=11,

=> 1/2 n(n+1)

=> 1/2 11(12)

=> 11*6

=> 66

If 1st=3, then 10th=66

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