There is a column of numbers 1.3.6.10.15.21. The nth number is represented by an algebraic expression containing n

A1=1
A2=A1+2
A3=A2+3
.
.
.An-1=An-2+n-1
An=An-1+n
A1+A2+...+An=A1+A2+...+An-1+ 1+2+...+n
An=1+2+...+n=n(n+1)/2

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