Now i know why...

Actually it came from arithmetic series where we could assume that

t1 = 0 = 0
t2 = 1 = 0 + 1
t3 = 3 = 0 + 1 + 2
t4 = 6 = 0 + 1 + 2 + 3

where a(initial number) a=0
and the d(differences) d = 1

we put it in arithmetic series formula Sn = (n/2)(2a+(n-1)d)

= (n/2)(2(0)+(n-1)(1))

=(n/2)(n-1)

tn = (n/2)(n-1) thanks!