Observe the number of the following column, 1.3.6.10.15.21... According to this rule, what is its number in 2004
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