Observe the number in the following column, 1,3,6,10,15,21. According to this rule, its number 2004 is

Observe the number in the following column, 1,3,6,10,15,21. According to this rule, its number 2004 is

a2-a1=2,
a3-a2=3.
a4-a3=4,
a5-a4=5
a6-a5=6.
.
an-a(n-1)=n.
The results are as follows
(an)-1=[n(n+1)/2]-1.
an=n(n+1)/2.
a2004=1002×2005
Can it solve your problem?

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