Observe the law of the following kind of numbers 0.1.3.6.10.15. What is the number 2006 (expressed by formula)

Observe the law of the following kind of numbers 0.1.3.6.10.15. What is the number 2006 (expressed by formula)

0.1.3.6.10.15
1 2 3 4 5
an-an-1=n-1
an-1-an-2=n-2
……
a2-a1=1
an-a1=1+2+…… +n-1=n(n-1)/2
an=n(n-1)/2
a2006=2006*2005/2=2011015

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